not've

Adverb

not (not comparable)

1. Negates the meaning of the modified verb.
   • Did you take out the trash? — No, I did not. I do not think it was my turn, was it?
2. To no degree.
   • That is not red; it's green.
3. Used to indicate the opposite or near opposite, often in a form of understatement.
   • That day was not the best day of my life. (meaning the day was bad or awful)
4. Used before a determiner phrase, a pronominal phrase etc. to convey a negative attitude (e.g. denial, sadness, anger) towards something.
   • Oh god, not that! Anything but that!
5. (informal, ironic) Used before a non-finite clause (especially a gerund-participial clause) or less commonly a determiner phrase to ironically convey some attitude (e.g. surprise, incredulity, amusement, or embarrassment) towards something.
   • Not me writing example sentences again.

See all 9 meanings of not