"time to fuck now"
This is a variation of "ta ta for now" and is mainly used as an inside joke after both parties have discussed the possibility that "ttfn" could easily stand for "time to fuck now".
(wrapping up a typical txt msg with a raging slut):
so u performed n a queef fetish porno with my dad 2 weeks b4 we met? ppl were right aboot u... rot n hell n TTFN, whorebag!!