(transitive, idiomatic) To know (something or someone) very thoroughly.

(idiomatic) To the best of one's knowledge, as far as someone knows about the topic, subject, aspect, or situation.

(of the giving of information) remaining secret or hidden until it is needed to be known (by a particular party).

(usually in the past tense) To have known a famous person before he or she was famous.

(idiomatic) A phrase used to get someone's attention before announcing something.

The name of something the speaker does not want to mention, but which is known, or very likely to be known, to the listener. It can be plural (as in "... the you-know-whats").

(chiefly US, in negative constructions) To know something, even if only a little, about. / (chiefly US, idiomatic) To know nothing, or almost nothing, about.

(intransitive, informal) To be knowledgeable in a particular field.

(idiomatic) Alternative form of wouldn't you know it

Used to speculate about a slight possibility.