Contraction of is not. (negative auxiliary)

(colloquial, nonstandard) Something or someone that is defined by the lack of the characteristic being discussed.

Observes, with some surprise, that a specified thing or person is present.

plural of isn't

(informal) Used to ask the listener for confirmation of or agreement with a statement; right.

An answer in reply to a yes-no question, indicating there is no simple "yes" or "no" answer

Isn't it true? / Isn't it that way?

Alternative form of it's not even funny

Alternative form of it's not even funny

(referring to a document) Used to indicate that (a document) was just issued.