Usually used when parents take away cell phones due to severly high phone bills.
(Yeah this is from the commercial)
Mom: I'll tell you...what's...SNF...
Girl: "IDK, my BFF Jill?"
Mom: "Well tell your 'BFF Jill' I'm taking your phone away."
"I'm taking away your phone for a huge bill."
"I'll tell you what's.... snf........."
Girl: Oh my god! I'm freaking out of gum.
Guy (valley accent): Uhh! TISNF!
q.v. NBD (no big deal) and BFF (best friends forever).