Someone who talks about other people behind their backs. What they say usually isn't true and is just what they think they hear from other people or they just made up themselves.
Shit-Talker: Hey my friend Jessica told me that you are hotter than her boyfriend.
John: Oh yeah?
John: So your friend told me that you think I'm hotter than your boyfriend.
Jessica: That bitch is lying. I swear on my life I never said that. That bitch is such a Shit-Talker.