A rip-off is when someone deliberately overcharges for something they're selling, or steals someone else's idea. A movie that uses the exact same plot as an earlier movie might be considered a rip-off.
Rip-off is an informal term for a theft or fraud of some kind. If you claim you're selling rare, antique postcards, but they're actually cheap copies you made at home, that's a rip-off. And if you copy an artist's design, print it on a t-shirt, and sell it, that's also a rip-off. First used in the 1960s, rip-off comes from African American slang, from the vernacular rip, "to steal."