... Not! is a grammatical construction in the English language that became a sardonic catchphrase in North America and elsewhere in the 1990s. A declarative statement is made, followed by a pause and then an emphatic "not!" adverb is postfixed. The result is a negation of the original declarative statement.
According to the above, the phrase "He is a nice guy... not!" is synonymous to "He is not a nice guy". Whereas the latter structure is a neutral observation, the former expresses rather an annoyance, and is most often used jocularly.
Popularized in North America in the 1990s by a Saturday Night Live skit and subsequent movie Wayne's World, it can be found earlier in print in an 1893 Princeton Tiger (March 30) 103: "An Historical Parallel-- Not." A 1905 usage is in Dream of the Rarebit Fiend by Winsor McKay. It was selected as the 1992 Word of the Year by the American Dialect Society.
The "Not!" catchphrase was the basis of a scene in the 2006 movie Borat where a lecturer in humour attempted to explain the grammatical construction to Borat with limited success.
The comedic utility of the word became apparent, again, in February 2011 in Canada, when a minister of the Crown, Bev Oda, was implicated in a scandal that saw her deny knowledge of the insertion of the word "not" on a legal document denying funding to a respected non-profit organization and offer no explanation as to how her signature appeared on the document. Normal procedure would have been not to sign the approval document. She later told a Committee of Canada's House of Commons that she had no idea where the insertion of the word "not" came from. Weeks later, she claimed that she ordered its insertion. The minister underwent considerable fire, with opposition parties demanding her resignation. She resigned on July 3, 2012.