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1.
A mountain Bike with 29" wheels rather than the more traditional 26" wheels.
Oh man, My XXIX is a 29er! It eats up bumps and is a monster.
by Bun E April 05, 2007
 
2.
Loose chick. Ok, this is a little out there, pay attention. Stock market crash on Black Tuesday in 1929 started the Great Depression. A 29er goes down faster than the stock market on Black Tuesday.
Hey, dude, can you give me your sister's number? I heard she's a real 29er.
by Bthomps October 21, 2004
 
3.
A new type of mountain bike, that is pretty much the only real type of mountain bike in existence (sometimes with 29 inch wheels, sometimes not).
"What kind of bike is that?"
"29er, f'yeah!"
by saxproaj October 09, 2007