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

(idiomatic) To have no common sense; to be ignorant of the most basic facts; to be very confused.

(idiomatic) To know or recognise someone at all.

(colloquial, childish) A retort to a negative accusation, implying that the accuser shares the fault.

(idiomatic) To know all there is to be known about a certain discipline.

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

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

(idiomatic, colloquial) suddenly, out of the blue.

(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