Used to ask the interlocutor whether or not he/she has met the speaker before.

Used to introduce, or alluding to, an apt adage.

(UK, Australia, New Zealand, South Africa, idiomatic, informal) To know; to be acquainted with (a person).

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.

present participle of know someone in the biblical sense

(idiomatic) To have sex with someone.

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

Alternative form of know which end is up

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