/not/ , adv.
1. (used to express negation, denial, refusal, or prohibition): You must not do that. It's not far from here.
2. U.S. Slang. (used jocularly as a postpositive interjection to indicate that a previous statement is untrue): That's a lovely dress. Not!
[ 1275-1325; ME; weak var. of NOUGHT ]