He'd a French cocked-hat on his forehead, a bunch of lace at his chin, A coat of the claret velvet, and breeches of brown doe-skin; They fitted with never a wrinkle: his boots were up to the thigh! And he rode with a jewelled twinkle, His pistol butts a-twinkle, His rapier hilt a-twinkle, under the jewelled sky.
A highwayman, put simply, is a guy who robs people on highways. The archetypal highwayman who is usually invoked by the word was found in Britain between, say, the years 1500 to 1800, although the same basic stuff went on elsewhere and elsewhen. They interrupt the journeys of rich people riding in coaches to say things like "your money or your life!" and "stand and deliver!". Standard gear seems to include a black outfit (possibly including a hat with a feather in it), a sword-and-gun combo, and perhaps a Domino Mask and above all a horse since that allowed them a quick escape. Armed robbers who weren't mounted were known as footpads.
At times, highwaymen were seen as glamorous. For various reasons (including the fact that they rode horses) they were considered a cut above common bandits. A proper highwayman, instead of being scruffy and furtive, was dashing and debonair - truly the Gentleman Thief of armed robbery. Some of them were built up as folk heroes ("...Just Like Robin Hood!"), and they have also been stock Love Interests in romance novels (perhaps because All Girls Want Bad Boys?). In certain types of story, it's also quite likely that secret identities will be involved - voluminous cloaks and nocturnal tendencies make it relatively easy for a prominent Rich Idiot with No Day Job to conceal who they are, or for a woman to avoid being known as such. Popular in The Cavalier Years, where the English Civil War is often blamed for their being forced to take up the occupation.
Highwaymanning became less attractive as a career with the development of toll roads (which are older than some people realise), steam trains (which get robbed under a different trope), and organised police forces. In works written recently, highwaymen tend to appear as parodies or deconstructions more often than they are played straight.
Not to be confused with the country supergroup of Waylon and Willie and Cash and Kris, though they do sing about being one (for the first verse of the song anyway)...
In particular the one in Lords and Ladies who holds up the wizards' coach and gets turned into a pumpkin, and the one in Carpe Jugulum who holds up the vampires' coach and gets drained. I think at least one of them also uses the "Your money and your life!" variant.
Casanunda, dashing swordsman, gentleman of fortune, and dwarf, has occasionally been a highwayman, although he finds it hard to get taken seriously. People say "I say, it's a lowwayman! A bit short, are we?" and he has to shoot them in the knee. He generally tells his targets to "Kneel and deliver".
Both books also have Casanunda demonstrating how sensible highwaymen get through such situations - by making friends with the wizards in the first one and staying the hell away in the second.
Rafael Sabatini wrote many stories about highwaymen, including several concerning the fortunes of a charming rogue who called himself "Captain Evans". (And, well-separated over the course of his career, at least three variations on a plot in which a clever but unpleasant person gets the better of a highwayman, robs him, and then gets caught red-handed with the loot and arrested as the highwayman.)
The Toby Man by Dick King-Smith is a childrens book about a young boy who becomes a highwayman with the help of talking animals.
One of Poul Anderson and Gordon R. Dickson's Hoka stories mentions that one of the Hokas has taken to dressing up as Dick Turpin and gets hanged every week. (Hanging doesn't actually kill Hokas; it's just one of the many things they adopted from human history and pop culture.)
Henry Fielding included these in some of his writings. Two examples are a highwayman who tries to rob the title character of The History of Tom Jones, A Foundling and is easily overpowered, but uses a sob story to convince Tom to not turn him in. Also, a bunch of characters in Jonathan Wild, which is a deliberately heavily fictionalized biography of an actual guy.
Patricia C. Wrede's Mairelon the Magician had a self-styled druid of dubious competence attempting to rob a coach filled with professional criminals in an effort to get his hands on an enchanted platter he wanted to use for a ritual (which the people in the coach didn't even have). He fails miserably.
Ratcatcher, the first novel in the Matthew Hawkwood series, opens with a pair of highwaymen robbing a coach and killing a naval messenger. The documents they steal are what drives the plot.
The eponymous robbers in the children's book The Three Robbers by Tomi Ungerer. The story was made into a six minute animated short in 1972, and into a full length animated movie in 2007.
Steven Brust's The Phoenix Guards series includes a number of highwaymen. One of the main characters also becomes a famous highwayman.
In the first series, Blackadder assembles the seven most evil men in the kingdom, one of whom is a highwayman. He uses the "your money or your life" line, but once he has the money, corrects the "or" to "and".
In the third series, Blackadder himself becomes a highwayman due to financial difficulties. One of the people he robs has a daughter who'd happily entertain the idea of being seduced by a dashing highwayman, but Blackadder isn't interested. Also featured is The Shadow, who gets the Just Like Robin Hood treatment from the population at large. The Shadow turns out to be a) a highwaywoman; and b) the same person who the prince regent is preparing to marry.
In Doctor Who episode "The Visitation", Richard Mace. He declares he is really an actor forced to this.
Help Im A Teenage Outlaw is a British show about three well-intentioned (but not necessarily competent) outlaws during the English Civil War.
Monty Python's Flying Circus has the highwayman Dennis Moore, who isn't very good at it. Most of his efforts involve breaking into fancy parties and stealing lupins; after he works out what he is doing wrong he redistributes wealth in such a way as to turn the poor downtrodden people into the new rich overlords, after which he tries to equally divide up the belongings of the people he robs.
In the Star Trek: The Next Generation episode "Family", Jean-Luc Picard accuses his nephew Rene, who hasn't seen him since Rene was an infant, of being a highwayman when he greets Picard on the way to their family vineyard in La Barre, France.
The first verse of the song "Highwayman" by Jimmy Webb, which became the signature song of the country super group The Highwaymen, deals with a highwayman of this type.
The English folk song "Reynardine" is about a girl who gets seduced by the titular highwayman.
Running Wild song "White Masque" depicts a folk hero type, who robs lords and marquises.
"Stand & Deliver" by Adam Ant is made of this trope.
Loreena McKennitt sung an adaptation of Alfred Noyes poem in her album "The Book of Secrets"
The Irish folk song "Whiskey in the Jar" is about a highwayman who is betrayed by his woman.
Another Irish folk song, "Brennan on the Moor" is a classic of the genre popularized by Burl Ives and the Clancy Brothers.
The Australian song "The Wild Colonial Boy," also known as "Bold Jack Donahue," and its many, many variants.
Another Australian song, "Waltzing Matilda", is about a highwayman who gets hanged.
The Far Side parodies the Wild West stagecoach version in "Semi-desperadoes":
"Throw down that strong box or I'll blow your head off!... Well, I'll wing you for sure!... Okay, maybe I'll just climb up there and give you a good Dutch rub."
Dick Turpin was a real highwayman who became famous for his mostly-fictional exploits, often being given the Robin Hood treatment. Alleged cars are sometimes named Dick Turpin, because they hold up traffic. (One example: Newt's car in Good Omens.) Your choice whether or not you think that's relevant. His modern reputation is a major Historical Hero Upgrade, as while lots of highwaymen were known as gentlemanly in their own time, his contemporary reputation was as a cut-throat.
In a similar vein to Turpin was William/John/James Nevison, a seventeenth-century highwayman who was probably nearer to an anti-hero but was later upgraded to being Just Like Robin Hood. Although Turpin is credited with the famous ride from London to York, it seems more likely that Nevison actually achieved this feat, and it was later ascribed to Turpin by the latter's biographer.
Black Bart (Charles Bolles), a stagecoach robber of the American Old West.
Highwaymen are a character occupation choice in Warhammer tabletop RPG, complete with horse and classiness. Ironically, one of the base occupations best suited to enter the class is the road warden, a horseback riding, gun-toting patrolman.
The 2012 version of the Iron Kingdoms RPG has the Highwayman as one of its careers, starting with a horse, mask and enough cash to get a decent gun and supply of ammo as well as abilities focussed on ambush tactics and firing from horseback. As the game requires a character to pick two careers at character creation, this can allow for some interesting combinations.
Randomly-generated Khajiit highwaymen show up in The Elder Scrolls IV: Oblivion, though they're not very gentlemanly; completing certain quests, triggering a one-use Good Bad Bug or actually being poor (defined as carrying less than 100 gold and wearing clothing worth less than 10 gold combined) means there's only a chance that they won't attack you. They're also a bit infamous in the fandom for always demanding 100 gold from you, even if (due to Level Scaling) they're wearing expensive Glass armor that they could sell for way more.
Morrowind featured a true gentleman robber. So much of a gentleman, in fact, that the associated quest involves pairing him up with his latest victim (she didn't know where he went, so you need to find him and bring him a message, and he didn't think himself worthy of her, so he didn't dare go and find her again).
The Classic Disney ShortThe Robber Kitten is about a kitten who dreams of being a highwayman. He runs away from home and finds out the hard way how unglamorous and dangerous it is to be one.