When you try to break up with your significant other (or actually do) but continue to sleep with them afterwards; normally for lack of a new sexual partner.
Is that Ashley in your bed mate? Well that was a total failcut.
To take something small, that doesn't quite qualify as a theft. Probably from the Danish "skæv" or the Dutch "scheef", both of which are pronounced similarly, meaning "askew, or not quite right'. To change an item's ownership without permission, but only something small and of little worth.
"I skeefed an apple off the neighbor's tree." "I skeefed some chips outta your bag when you looked away." "Don't skeef my chair when I go to the bathroom."