(idiomatic) To possess confidential and incriminating information about a person's or organization's misdeeds or other secrets.

(US, vulgar, chiefly in the negative) To have the most basic level of intelligence or common sense.

Alternative form of better the devil you know than the devil you don't know

Used to show a relevant fact that is unknown to a person, or that the result or sequence of events differs greatly from what one had expected with the little knowledge he or she had about the situation beforehand.

One will demand or expect justification; used in threats.

Alternative form of it takes one to know one

(idiomatic) To clearly understand one's own feelings, intentions, preferences, etc; to know precisely what one wants; to be decisive or determined.

Alternative form of know which end is up

(intransitive, idiomatic) To be experienced and knowledgeable. / (transitive, idiomatic) To be very familiar with; to have a significant understanding of.

(idiomatic) To possess sound judgment or common sense; to have a clear understanding of a situation.