Not!

not

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!
Farlex Dictionary of Idioms.

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!
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

not!

See i don't think so.
The Dictionary of Clichés by Christine Ammer
See also:
  • not
  • not (oneself)
  • Ooh, get you!
  • my bloody oath
  • bloody oath
  • oath
  • bonus
  • Bonus!
  • oh really
  • Oh, yeah?
References in classic literature
Fair mirror and true- thou liest not! Thou hast no end to gain -- no heart to break -- Castiglione lied who said he loved -- Thou true -- he false!
Thou heardst it not! -- Baldazaar, speak no more To me, Politian, of thy camps and courts.