When a man's cock accidently slips out of whatever he's fucking. Can happen if he misjudges his stroke, if his partner is too loose or wet, if his cock is short, or if he is unfamiliar with a position (usually doggy-style).
That girl was wide open, man. Way too much slippage, and right when I was 'bout to bust. (Note: Generally not a word used in normal conversation. 'Slip' would be the most commonly used variant.)