Acronym for "thanks so much". One of those expressions that, depending on how used, can be sincere or sarcastic in its meaning. To further confuse, can be used in both lower and upper case.
Sincere: "Hey man, looks like we were gonna run out of booze tonight after all! TSM for bringing that kegger, dude!"
Sarcastic: "I TOLD you I wasn't using birth control!! TSM!!!"
Uncertain: "Great birthday present, Harvey. tsm"