The Pimped-Out Dress of the 18th century. Powdered wigs, knee-breeches, ridiculously overdecorated dresses, wide-brimmed hats, beauty patches on powdered faces, just to name a few. They were predominant in Europe, the most in France, and the Americas, from the time of Louis XIV to The French Revolution. During that period, they were elegant and stylish, yet very ridiculous in extremes. The person who killed the fashion? Marie Antoinette, of all people, who was painted wearing a simple (in comparison) summer shift. (The difference was so extreme that at first many people thought Marie had been painted in her nightgown.) As the French Queen, Marie of course wore her share of pimped out dresses - because that was her job - but she preferred more simple clothing. Interestingly, the very Revolution that cut off her head took credit afterwards for the change.
Bell-bottom jeans went from being cool in the 1960s and 1970s to being extremely uncool in the 1980s. Since the 1990s the fashion industry has tried several times to "bring back" bell-bottoms and every time they've failed to catch on for very long. These days bell-bottoms are mostly worn only by sailors and members of hippie and stoner subcultures.
For most of the 20th century, hats (not caps) were an essential piece of clothing for men, to the point that going out in public without one made you look strange. But around the late 1950s and early '60s, wearing a hat became entirely optional. By the '70s, wearing a hat was seen as old-fashioned. These days, apart from the aforementioned baseball/trucker caps, the men's hat is almost never seen in public. The reason that hats fell out of fashion is often debated, but some of the more popular theories include John F. Kennedy's supposed refusal to wear a hat during his inauguration (he did wear a hat) and the rise in popularity of sunglasses. It's also worth mentioning that dress styles in general have gotten a lot more casual over the last 50 years; prior to the '60s it was standard for men to always wear a suit in public. These days jeans and t-shirts have become the norm.
The military draft, in place from 1940 to 1973, had a lot to do with men's hats going out of fashion. Having to wear a hat under threat of punishment if you don't - ranging from confinement to quarters to loss of pay up to and including incarceration - is almost guaranteed to give you a bad case of "anti-hat-itis".
The problem with that theory is that in the military the headwear people forced to wear are generally caps and helmets, not hats. Another is that the wearing of hats was in and went out of fashion at the same time in countries which had the draft longer, such as France and Germany.
Slap bracelets were a fashion accessory that consisted of a flexible steel band covered by a layer of fabric or plastic that came in a large variety of colors and patterns. People would slap the straightened band against their wrist, causing it to spring back into a curve that encircles the wearer's wrist. For a few years in the late '80s and early '90s, slap bracelets were a popular fad among children and teenagers. Then schools started banning them after reports of students being injured due to improper use. Once schools started banning them, slap bracelets quickly fell into obscurity.
As per the Popularity Polynomial, they're making a sort of comeback as wristwatches. Having a thicker silicone coating might help.
For a while in the 1990s, barbed wire tattoos (that wrap around the upper arm) were considered cool because of celebrities like Pamela Anderson who got them. But after a while they became so widespread and mainstream that they're now considered one of the more unimaginative types of tattoos.
The fedora seems to be heading in this direction. After spending many years as an iconic piece of Good Guys Garb (there's even a trope about it), the hat and its derivatives have developed an increasing association with the creepier varieties of nerd and hipster (likely in imitation of said trope), as well as an increasing association with He Man Woman Haters and Hollywood Atheists. Say "fedora" these days, and the quickest thing to pop into most people's heads is less likely to be Indiana Jones or Humphrey Bogart and more likely to be someone like this guy◊.
Though the hat most people on the internet seem to think when you say "fedora" (including the one in the picture above) is actually an entirely different hat called a trilby. The actual fedora is still mostly associated with Humphrey Bogart or Indiana Jones (mostly because it's not very commonly worn by anybody, likely due to looking rather silly unless you're properly dressed).
Watches came dangerously close to becoming this in the wake of cell phones. With a cell phone giving an exact digital display of the time and didn't have the consequences of having a battery run dead or needing to be wound up, watches became seen as an unnecessary extravagance. However, watch manufacturers wisely went upmarket and changed their marketing from "practical way to tell the time" to "stylish status symbol" (hence the Product Placement of Omega watches in James Bond films) and seem to be doing reasonably well. In addition, smartwatches like the Pebble have positioned themselves as companions to smartphones, being more convenient and socially acceptable to check rather than pulling a phone out of your pockets, and also able to do things like receive text messages and run simple apps.
Wrap around ear wire framed eyeglass frames.
Suspenders are nowhere near as prevalent to hold a man's pants up as they once were, having been almost completely eclipsed by belts (one has to wonder if Steve Urkel had anything to do with it.) They're still being made, sometimes with decorative patterns on them, but on the few occasions you do see them, they're either worn by hipsters (which, again, can't help their reputation) or used to deliberately invoke a more classic throwback look.
The "bald spot in the midst of normal-length hair" look. Now that George Constanza and a slew of TV dads during the '80s and '90s have made this look seem ridiculously uncool and unsexy, and the "bald-with-a-small-bit-of-fuzz-on-the-side" look is considered to be rather sexy/manly (thanks to such celebrities as Bruce Willis, Michael Jordan, and countless hard rock and heavy metal musicians), you'd be hard pressed to find a balding man under the age of 50 who still maintains normal-length hair on the side of his head.
For that matter, toupees. Now that hair implants have become mainstream and full male baldness has become not only more acceptable but also fashionable (see above), toupees are pretty much a thing of the past. Plus, unless you had a ton of money to spend, they generally looked horrible.
Even if you DO have a ton of money to spend, they can look horrible. See: Donald Trump.
"Flat-top fade" haircuts among African American men (think Fresh Prince-era Will Smith), though in true Popularity Polynomial fashion, this one has started to make a comeback.
"Jheri curls" have also completely vanished and is now associated with the big hair of The Eighties. They were messy to maintain and required a lot of hair-damaging chemicals. The similar "S-Curl" for Black men, while still seen in stores, is also not so popular anymore.
The "business at the front, party at the back" mullet haircut. From the early 1980s to the mid-'90s it was actually considered stylish, and many male celebrities, including Bono and Mel Gibson (in the first Lethal Weapon film), wore their hair in this fashion. But by the late '90s, the mullet was considered extremely uncool and heavily associated with stereotypes of "white trash". These days, whenever a character is portrayed with a mullet, they will inevitably be portrayed as a redneck or a lowlife (or at the very least he'll be portrayed as a Disco Dan who is hopelessly out of touch with modern trends).
The crew cut: Often associated with the conservative style of The Fifties. Even in the military, they are becoming rarer with men opting instead for "even buzzcuts" or shaved heads. The counterculture of The Sixties was the death blow for it as a civilian haircut, with young men favoring longer hair. In The Seventies, even the squares had longish hair.
Full beards among young men. Goatees and soul patches are more common. Also, mustaches seem now to be increasingly reserved for older men over 40. In the U.S., mustaches without any other facial hair seem to be associated with mature men and is also highly perceived as a conservative look. The so-called "law enforcement mustache" that used to be seen among policemen is seen less frequently. Due to the strict police department regulations regarding facial hair and the need to maintain the mustache within strict regulation length is one reason it is no seen more often. Outside law enforcement, the same style of mustache has also become associated with 70s male porn stars and is often a source of ridicule.
Male ponytails. Especially if the top is balding.
Curly afro style among White men. Sometimes known as the "Jew Fro" although one need not be Jewish, it was prevalent due to more curly hair amongst men of Jewish or partially Jewish heritage. May be accomplished with the help of perms. Former figures known for this style include Art Garfunkel, Gabe Kaplan of Welcome Back, Kotter fame, Tom Baker and Colin Baker during their respective Doctor Who tenures, Napoleon Dynamite, Elliot Gould, and Slim Goodbody during The Seventies and The Eighties.
The male sideswept fringe (or 'wings'). Originally associated with the Emo subculture, the cut became popular in mainstream male fashion in the mid-to-late 2000s. By the middle of the The New Tens, public figures who had previously worn it - perhaps most notably Justin Bieber - had deserted it in favour of a clean-cut style.