1. Not kidding! - Means the speaker is not kidding. It is the antonym of the acronym: Jk, which means just kidding. Therefore, Nk means that the speaker was not kidding instead of really kidding. Usually used as an insult or otherwise.
2. No kidding? - Means the respondent is asking if the speaker is not joking.
3. No Kissing. - Means the kind of thing you do not want your significant other, girlfriend or boyfriend to say during intimacy or otherwise
1.
Timothy: Your girlfriend is a freak...
Francis: Are you kidding?
Timothy: Nk.
2.
Francis: No kidding?
Timothy: I meant not kidding moron.
3.
Joan: Nk tonight...
Timothy: Why!!?
Joan: You have bad breathe sorry.