verb - A sexual position involving a small bed (i.e. a single) in which the female or effeminate partner during intercourse lies down on the bed, facing up as if sleeping normally and, to "keep balance" the male or masculine partner during intercourse lies down on top of the other person, but is instead perpendicular to the bed so their upper and lower body are hanging over the side of the bed. This leaves a sizable portion (estimated at "50 percent") of the body not on the bed, and thus is where the name comes from.
Guy 1: Hey man, I heard you were going to hit up that hottie you hooked up with the other day; did you get any last night?
Guy 2: I had my whole day planned out so I could mine her gold mine all day, but her bed was only big enough to 50 percent on and my MASSIVE biceps would put me off balance, so I couldn't do it.
Guy 1: Yeah, right, you're five pound weights really attest to your muscles, huh? There never was a hot chick in the first place.