ILY stands for I LOVE YOU and TNT literally stands for the words "the number two".
This is easy to use when making fun of acronyms i.e. ily.
Girl 1: ILY!
Girl 2: ILY2!!
Girl 1: No, you can't say "ILY TOO".
Girl 2: I DIDN'T.
Girl 1: Yeah, you did.
Girl 2: Hmmm... How about "ILY the number 2"?
Girl 1: It's an acronym. NO.
Girl 2: Well, then I'll make "the number two" into an acronym - TNT. ILYTNT.