Slang for sex. Date's back to "the Fresh Prince of Bel Aire" where buck-forty was not recognized with out it's visual component. Moving hips in this pattern: left,right, back(stick your butt way out), and FORTH (thrusting hard) You say: Nickel, Dime, Quarter, DOLLAR! Substitued later, the younger generation shortened the phrase to buck-forty.
We Buck-fortied ALL night long!
by mos def June 13, 2003
hitting one hundred and forty miles per hour in your car/truck/motorcycle
man DAMN we just hit a buck-forty
by evan March 29, 2005