Just like computer programmers, people who dress up like Klingons, and the French, bicyclists have their own super secret special language. And just like the French, they will howl mercilessly if you mispronounce words (say "derailleur"... go on, say it!!), or worse, don't know what they mean. That's why we're happy to provide this beginner's list of cycling terms.
Brain Bucket: That's a helmet! (Durrrrrrr.)
Fred: If someone calls you a "Fred," that means you're either wearing a fancy-pantsy Spidey-suit (see below) and/or riding a bike that far exceeds your abilities or need. For example, see half of the people currently whizzing down the Springwater Corridor.
Fixie: A fixie is a single-speed fixed-gear bike that cannot coast—meaning the pedals will be in motion when the bike is in motion. Advantages: With fewer parts than geared bikes, the fixie is lighter and more efficient. Disadvantages: You look like a dick. It's neither fair nor logical... but there it is.
Rad Dads: Grownups who ride around on those little BMX bikes. Often seen without brain buckets and perhaps under the influence of meth.
Squirrel: Someone who rides all "squirrelly" without regard for those around him/her. Example: Rad Dads on the Esplanade.
Hasselhoff: A shirtless rider.
Wall Street: Someone who commutes to work on their bike wearing a business suit—which is fine if your job is being a Jehovah's Witness.
Lovebirds: A pair of goo-goo eyed lovers who ride (often squirrelly) side-by-side, slowing down traffic, and making everyone hate them and their stupid love.
Chainring Tattoo: Also known as a "Rookie Mark," it's that smudge of grease that's on the inside right of your pant leg, on your calf. That's why you see people walking around looking like...
The Gimp: The person who rolls up their right pant leg to avoid a chainring tattoo, but forgets to roll it back down.
The Salmon: A bicyclist riding the wrong way up a one-way street, or "swimming upstream."
Endo: Flipping end over end, as in, "That squirrel in a Spidey-suit totally made me do an endo. Good thing I was wearing my brain bucket."
The Annoying Samaritan: A driver who has the right of way at a four-way stop, but waves you through even though it was his turn, but you've already come to a complete stop, so when you try to tell him to go, he smilingly refuses and says "after you," and oh, for the love of Christ... JUST DRIVE YOUR FUCKING CAR!!
Spidey or Super Suit: Spandex jerseys, shorts, or bibs that match a little too perfectly—though they do give the wearer the power of "indickability."
Organ Donors: People who blow through red lights and stop signs. Often seen without brain buckets.
Minotaur: A person with calves the size of a Minotaur.
Spidertaur: A person with calves the size of a Minotaur wearing a Spidey suit.
Derailleur: When shifting, the derailleur moves the chain from gear to gear, and is pronounced "de-RAIL-er," or "de-RAIL-yur" (if you're a dick in a Spidey suit), or "day-RYE-EUH" if you really want to be French about it. And you don't.
La-Z-Boy: A recumbent bike rider.
Bike Ninja: Dark clothes, no reflective gear, and sneaks up at night scaring the bejeesus out of you.
Zombies: Pedestrians who use bike lanes as an "extra" sidewalk, or wander aimlessly into your path, or let their kids or dogs run out in front of you. In other words, just pedestrians.
Check out bikehacks.com's "Dictionary of Bike Commuter Slang" who provided a few of these, and has many, many more!