Acronym for Toxic Guy Syndrome. When a guy, despite having a girlfriend, tends to let his lower head's thoughts overpower his upper head's thoughts. Basically when a guy thinks with his penis instead of his brain.
Guy 1 : Hey, man I haven't seen Stephanie around in a while.
Guy 2 : Yeah, bro, I think I have TGS.
Guy 1 : Oh, you cheated on her?! With who?
Guy 2 : Some hot piece of ass I saw at a party last month.