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negative particle, mid-13c., unstressed variant of noht, naht "in no way" (see naught). As an interjection to negate what was said before or reveal it as sarcasm, it is first attested 1900; popularized 1989 by "Wayne's World" sketches on "Saturday Night Live" TV show. To not know X from Y (one's ass from one's elbow, shit from Shinola, etc.) was a construction first attested c.1930. Double negative construction not un- was derided by Orwell, but is persistent and ancient in English, popular with Milton and the Anglo-Saxon poets.
What has just been stated is emphatically not true: Millions of animals in experimental labs die annually. Shock value? NOT!/ Hooray for Pat Buchanan, not!/ Dan Quayle has already filmed a commercial declaring Murphy Brown his favorite show ...''not'' (1990s+)