An agreement between 2 people prior to fucking that limits the man's liability if the women becomes pregnant and wants to keep the child,and the man doesn't!
Buster and Trixie have been knocking it out on a regular basis. Trixie lyingly told Buster that she was "on the pill". Trixie got pregnant and wanted to keep the child. Buster is screwed because he didn't get a pre-fuckual agreement. Buster will be continually fucked for the next 18 years, 9 months.