1. slang Someone who pursues a celebrity or performer, most often a musician or member of a band, typically by following their performance tour, in order to gain intimate access to them, and especially to pursue a sexual relationship or encounter. The term was originally applied primarily to women who pursued rock musicians in such a way. Aunt Ida was a groupie for Grand Funk Railroad in the '70s? Are you for real?

2. slang By extension, a passionate fan of someone or something. I'm a real groupie of that author, so of course I'm going to her reading tomorrow night.