A person (usually one of your friends) that says they will do something with you, but then "stabs you in the back" and goes and does something else. (A "cunt kniver" is someone who will do this frequently with their girlfriend).
When someone you madeplans with disappears and becomes a Knifer / Kniver
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."