look up any word, like b4nny:
 
1.
When gambling on an event whether it be a card game, at a casino, at a race track, etc., two or more people may choose to place a seperate bet away from the main pool of betting.
Lets say Joe, Bob, Chris, Tom, and Becky are playing a card game and they are all betting $10 a hand. Tom and Joe may get a little extra competitive and decide to place a side bet between just the two of them. They will bet the originally agreed upon $10 on a game with everyone at the card table involved and they will bet, lets say another $25 just between the two of them.
by Douglas Teel October 19, 2007
 
2.
Someone you keep on the side even though you have an interest in others.
I'm just your sidebet.
by EllaMint July 18, 2006