What is the singular possessive of not?

Singular forms and possessive forms are terms for nouns and pronouns.

The word 'not' is an adverb, a word used to modify a verb, an adjective, or another adverb. Examples:

modifies a verb: This is not a good day.

modifies an adjective: He is not my boyfriend.

modifies an adverb: We are moving but not very far.