Ascended Fanon: The Deimos Clan Heavy mech was originally a non-canon mech introduced by Mektek's free re-release of MechWarrior 4: Mercenaries. It was later made canonical with the release of Technical Readout 3085
Development Hell: Handbook: House Kurita, and to the lesser extent Interstellar Operations.
Here's a fun drinking game: Go to the official Battletech forums. Search for "ilClan." Take a shot every time a post asking what the progress on it is appears. Since this is a book that's been awaited for years, you may need to stop, either voluntarily or involuntarily after a short while. This may finally be subsiding as Word of God says that the reason for the delay is they're still trying to tie up loose ends and come up with a new, post-ilClan metaplot to avoid the deadly question "And Then What?". Eventually, the delay got so long they decided to split it into two parts, with the first part becoming the Shattered Fortress which was released on August 2018 and the planned second part retaining the ilClan name.
There's a lot of Fantastic Slurs against members of opposing Great Houses, due to bad blood from atrocities committed during the Succession Wars:
Crappies - a slur against Capellans and in particular House Liao members that are incompetent.
Fedrats - a slur against members of the Federated Suns, most commonly bandied by Capellans, Draconis Combine citizenry, and Taurians.
Bulls - a non-flattering way for Feddies to disparage Taurians due to their stubbornness and aggressive resentment of the Suns.
Snakes - A common insult fired at Draconis Combine citizens for perceived treachery and viciousness, in particular by FedSuns citizens from the Draconis March.
The more fanatical members of Clan Jade Falcon are often made fun of by everyone, commonly referred by such names as "green canaries", "bird-brains", and such. Thanks to their emblem of a purple eagle, members of the Free Worlds League get made fun of with similar bird-themed insults.
Coring - Taking out a mech by destroying the center torso.
Kneecapping - defeating mechs by taking out their legs.
Headchopping - taking a mech by a solid hit to the head/cockpit, scoring a lethal crit on the pilot.
Headchopper - a weapon suited for such hits - most commonly a AC/20, Clan ER PPC, or Gauss weapon, but anything that can one-shot a mech with a lucky head crit may be referred to as such especially if a player is skilled enough to keep landing headshots.
Can Opener - Often used to refer to powerful weapons that can knock off entire sections of armor in a single hit, such as the PPCs, AC/20, and the Gauss Rifles.
Critseeking - a general strategy of using a large volume of burst/volley fire weapons that can roll multiple times for critical hits, such that once you breach enemy armor you have a fairly good chance to hit something vunerable and disable them. Machine guns, LRMs, SRMs, LBX Autocannons (loaded with cluster ammo), and large batteries of Small or Micro Lasers are popular for this sort of thing.
Crit Padding - The practice of loading less-critical subsystems (like extra heat sinks and jumpjets) into a body component that has vital weapons installed, so critical hits have a chance to break them instead of either breaking your guns or torching your ammo.
Seatbelt Check - A dreaded piloting roll when your mech loses its balance and falls over. Failing it can mean pilot wounds and/or even death.
Lawn Darting - A dreaded counterpart to the Seatbelt Check, mostly used for ASF aircraft failing to remain airborne (due to AA fire from Autocannons loaded with Flak rounds or unlucky rolls), but sometimes used for either 'mechs failing to complete Jumpjet operations or LAMs (Land-Air Mechs) failing to stay aloft and faceplanting. More likely to total the unlucky victim due to Gravity being merciless in claiming her pound of flesh and steel.
Peeking - a very common defensive tactic of only exposing just the arm (and/or side torso) of your mech briefly so that you can fire at the enemy with as small an exposed profile as possible to minimise damage from return fire.
Poptart - a Jumpjet-based tactic of jumping just high enough to peek over the top of hills just long enough to open fire and then drop down before return fire can hit you. Under certain circumstances this tactic can be used to provide sighting for lancemates.
Torso Twist - A defensive move used against laser weapons in Mechwarrior Online in particular, but also mentioned in the fluff. Turning your torso in just the right way at the right time can prevent laser weaponry from burning a clean hole through your armor, by forcing the beam to sweep less dangerously across the armor and leaving a shallow furrow in the armor rather than penetrating it entirely due to focusing on a single point. Pulse Lasers are much harder to Torso Twist against due to their behavior.
Manmade Lightning - A term popularized by the Battletech writer Stackpole, it refers to the electrostatic discharge of the PPC weapon system.
Stackpoling - Due to the writer Stackpole's propensity for making 'mechs in his stories explode (for the Rule of Cool, of course) despite the canonical safety measures in Fusion Engines made to prevent that, this term was applied to the broad category of faults and damages that can result in said safeties failing, resulting in a silvery blast as the 'mech explodes from the engine failure and turning into a shrapnel and steam explosion. There are Critical Hit rolls for crits to the engine that can result in Stackpoling (and even if a 'mech doesn't explode from such crits, they are usually sufficient to trip the safeties and shut the engine down anyway).
Missile Boat - a common term for any mech configured for SRM and LRM spamming.
Flashbulb - a 'mech built with an all-energy setup, with no ammo-using weapons whatsoever.
Ammo Bomb - a 3025 'mech with a side/center torso-mounted ammo store with too very few other subsystems in the same section to absorb hits, rendering the ammo highly vunerable to critical hits that would result in deadly ammo explosions (Ammo explosions deal damage equal to the remaining unused munitions in the section struck by a critical hit, so a ton of machine gun ammo would go up like a cluster bomb due to all the bullets cooking off, for example). Among the infamously vunerable mechs include the Marauder 3R, which has only got crit slots for the AC/5 ammunition inside its side torso and nothing else, thus making crits to that section once armor is stripped (or a player lands a lucky Golden BB style Through-Armor Crit) extremely deadly. CASE technology made this phenomenon much less deadly, but ammo explosions remain a dangerous threat to a mech anyway since it will still destroy the affected section of the mech.
Laser Vomit - the Mechwarrior Online version of a Flashbulb mech.
Disco - A term used to refer to 'mechs running lots of pulse lasers, which has a strobe-like effect when multiple are fired at once in games like Mechwarrior Online.
Discoback - A nickname for the Hunchback 4P, that trades off the signature AC/20 of the baseline 'mech for a battery of medium lasers that hits harder but generates much more heat in turn.
Ghost Heat - A balancing mechanic in Mechwarrior Online where firing too many of the same kind of weapon at once will cause abnormally large amounts of heat buildup to dissuade players from relying exclusively on any single kind of weapon.
Bugmech - A common moniker applied to a number of light mechs and fast mediums (like the Cicada, Locust, Stinger, Wasp, and Flea), which are all Fragile Speedsters and have insect-themed names. Amusingly, the Grasshopper isn't quite a bugmech because it's a fast Heavy mech and is thus a tad overweight to be considered one of them (and likewise, the Crab is a Medium zombiemech and the King Crab is a 100 tonner Assault).
PBI - Poor Bloody Infantry, an unfortunately true turn of phrase to describe common infantry in a battlefield full of armored vehicles and huge battlemechs with intimidatingly large weapons.
Shield Arm - A common mech customization tactic of having one arm free of critical subsystems, so you can use it as a buffer against attacks by angling your mech to take hits there.
Zombiemech - a battlemech designed such that it can retain a large portion of its fighting ability despite taking grieveous amounts of damage, by using only- or mostly energy weapons(no ammunition explosions), putting weapons and heatsinks inside both side and center torsos as well as the head and legs in such a way that component loss would not suddenly cripple its entire ability to fight, and using Standard or Compact engines instead of Light and Xtra-Light engines (XL engines are mission-killed if the side torso structure of a mech is totaled, and Light Engines suffer a heat performance penalty when you lose one side torso section). So, much like the stereotypical Zombie, the only way to quickly put one down is to shoot it in the head(and if they have a torso-mounted cockpit, even that won't work!). 'Mechs like the Crab, Marauder 3D (which ditches all the ammo-based weapons for lasers), and the Roughneck are infamous for being such a kind of immensely durable mech, and their Structural bonus quirks in Mechwarrior Online only serves to emphasize this.
"Spooky Atlas" - an Atlas refitted with Stealth Armor (and ECM) and given a skeleton paintjob. Invented by MWO mechwarrior theB33f, the Spooky Atlas is basically a tech-based Phantom Mech in its playstyle - you sneak up behind an enemy with your Stealth Armor active and hold your fire until they turn and see you up in their face. Boo.
"Lurm" - an affectionate meme name for the LRM missile system courtesy of the MWO playerbase.
"Solahma Suicide Sled" - The Chippewa ASF, which has a measly 8 tons of armor for a heavy ASF. Similar terms apply to other Inner Sphere mechs and ASF that have dangerously thin armor for their tonnage. This term came about as a way to describe under-armored units native to the Inner Sphere by Clanners (often used in surprise, as solahma are a Clan thing).
"Nicky K" - Nicholas Kerensky, the son of Aleksandr Kerensky and the man who created Clanner culture. Don't actually address him as such in front of a Clanner.
"Tasha K" - a nickname for Natasha Kerensky, one of the famous named mechwarriors in canon and leader of the Black Widows of the Wolf Dragoons. Don't call her that to her face.
"Mad Max" - a nickname for Maximillian Liao, who became increasingly unhinged after Hanse Davion humiliated him and declared war at the same time during his wedding.
Real Life Writes the Plot: The reason why the Eridani Light Horse mercenaries were killed off, and in really gruesome way for that matter (Left to die in the in the crossfire of two clans). The longer version: Someone wrote a fic about the ELH backstory, and due to various circumstances, that fic was canonized by accident without paying the author. The fanfic writer sued, and the case would go on and off for nearly a decade until the developers, already wary after the Unseen debacle, decided it was not worth it and got rid of ELH. And while they were gotten rid of in the 3060s timeframe, even newer 3025-era products avoid more than token mentions of them.
Keep Circulating the Tapes: The series was never released on VHS, let alone DVD or streaming (considering the acquisition of the Saban Entertainment library by Disney in 2001, it's likely never going to see a proper release). Episodes can be found online, but all that's available are poor-quality rips of home tapings.