The smile that appears on a girl's face when she rides a bicycle. Riding along, rubbing their crotch against the seat makes them smile!
Possibly originates from "The Road to Wellville"(1994) but probably much earlier.
See Jane ride her bike.
See Jane's bicycle smile!
Jane sees Dick.
Look out Dick!
when a female is riding a bike, usually over rough terrain, and she slides the bike seat into her VJ. this will result in a smile on her face and an odd musk on the bike seat
Colleen "Nice new bike Vanessa"
Vanessa "thanks Coll"
Colleen "hey where's the seat, and why are you smiling so much?"
Vanessa "oh I'm performing a Bicycle smile, lets find some rougher terrain"