A guy/girl who becomes the test subject in a relationship. He/she had a crush on you, without knowing that you're just wanna play around with him/her. Usually ends with the guy/girl being dumped.
Watch the movie 'How To Lose A Guy In 10 Days' to get better idea
Guy #1: That jock
at school just wanna have fun with one of the cheerleaders. He doesn't intend to have serious relationship with her
Guy #2: Too bad the girl still fell in love with him
Guy #1: Oh well, there goes another crush test dummy