This is a slang term for re-raise.
SoundPoker Says: For example, assume in a hand of limit poker Mike is under the gun and he decides to raise $5. The action then falls onto John, who decides to re-raise Mike 2 bets to $10. The act of re-raising Mike's opening bet is known as a Backraise.
See Also: Action, Limit Poker, Open, Position, Raise, Re-Raise, Under The Gun