When an item, be it living (God forbid) or inanimate, gets stuck between a woman's breasts. Sometimes it's an accident, sometimes not. Most of the time it's an accident. If the female subject is with other females, they call out "BOOBY TRAP!". Sounds stupid but it's really funny...
During the act of intercourse, a man puts his penis inbetween the girl's breasts and proceeds to thrust up and down. The breasts (if big enough) should 'trap' the man there until he finishes. Hence - the booby trap.