Tg means "this guy". It can be used in pretty much any situation. This guy is an expression we use when someone does something idiotic or unbelievable.
friend 1: Hey have you heard? Friend 3 failed his exam even after we helped him study LOL
friend 2: tg...
1. Totally Gayed.
- Something bad that would probably only happen to you.
2. Tourette's Guy - Bad things always happen to the Tourette's Guy (pretty much the same thing as getting totally gayed)
Used as an abbreviation for "that's great", a phrase often used heavily dipped in sarcasm following an intensely boring or irrelevant anecdote made by another person.