Acronym for "testicle, left, suck it." Both the acronym and the statement itself should be used as a response similar to "suck my dick." If the full statement is used, it must be said slowly and with distinct pauses between each comma.
Nate: Yo, Jamal! You see all dem bitches I was wit last night?
Jamal: TLSI, Nate. TLSI.
Too late seen it. often used online when someone links you to an image what you've already seen.
Guy1 "Hey dude check out this cool image"