Acronym standing for Not That Nice. Can be used to characterize almost anything like a show, girl, etc. Mostly used while IM'ing chatting someone, otherwise, sounds retarded to actually speak.
Evan- "Yo, do you like that new Eminem song?"
Daniel- "Nah man, it's really NTN"
Person 1- "Ok...Sasha?"
Person 2- "Nope, NTN"