someone who only does something if they will benefit from it. Someone who is so two-faced that you might even call them Tommy Lee Jones.
Frank: Did you hear about about that whalen guy?
Jerry: No. What did he do?
Frank: He lied to my face about something, tried to sleep with my ex-girlfriend, then acted like his fake self in order to benefit from a situation.
Jerry: Damn. I can't think of a better way to describe him than calling him a whalen.