I first heard rip-off in the 60's, usually hip lingo pertaining to someone getting over on a drug deal, usually pot (selling it short of weight, cutting it with oregano, etc.). Then it rapidly came to mean any kind of conning, stealing, or getting over..."Are you ripping me off?" "He's a real rip-off!" "Man, what a rip-off!" "I just got ripped off!" How, or why, it was coined, I don't know. I do know that ripped or really ripped was another expression for stoned...and then any kind of inebriation. (and I think this missed our list!)