1. vulgar slang To reach orgasm, especially through sexual intercourse. Usually said of a male ("rocks" is a vulgar slang term for testicles). He has a reputation of being a tom cat just looking to get his rocks off. I don't want a serious relationship, just a guy I can get my rocks off with every now and then.

2. vulgar slang By extension, to find great enjoyment from something. There are a lot of online trolls who get their rocks off on insulting or annoying other users.