tkt is a French msn/chat short for "T'inquiètes", meaning "Don't worry/That's OK" or "I know what I'm talking about/I'm gonna make it.".
A: I'm not sure you're gonna win this contest.
B: tkt I'm sure I'm gonna.
French equivalent of "DW" (don't worry), litteraly abbreviation of "ne t'inquiètes pas" (means don't worry), which becomes > t'inquiètes > t'inkiete > tkt
Note: The "qu" becomes often "k" for the SMS/MSN french generation.
"tkt je le ferais"
(dw, i'll do it)