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An interjection negating the statement one has just made and thus rendering it sarcastic. Oh, yeah, I love getting a ton of homework on my birthday weekend. Not!
interj. Not really so! (A tag phrase added to the end of a statement, changing it from affirmative to negative. There is usually a pause before Not!, which is said on a level pitch somewhat higher than the sentence that comes before.) Of course I’m going to pay $100 a ticket to see a rock concert. Not!