Abbreviation of Totally Jerkable. TJ! basically means that you would totally jerk off to a hottie you just saw.
Usually TJ! is said in public so the girl your talking about doesn't understand.
As you say TJ!, you must make a hand gesture of jerking off to imply that this chick is indeed Totally Jerkable.
Pervert 1: "hey, did you see that hottie with the tits?, holy shit, makes me wanna comjizzle in my pizzle pazzle"
Pervert 2: "TJ!" *hand motion*