Americanslang for stealing or pocketing an item, often of dubious or minor value. Archaic , dates from the 1950s-1960s, especially popular on the West Coast in those days.
May have been replaced by "rip off" in the late 1960s, which became a much more broad term and eventually a noun as well as a verb.
"I'm not putting my beer down. One of these biker dudes might cop off with it."
"Where's my hat?"
"That chick you were talking to earlier. I think she copped off with it."
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."