Refers to someone one knows personally, not necessarily a close friend. Often used in contrast with strangers or in contexts of mutual recognition.
See also: 知人 (ちじん)
He is a famous photographer known to those in the know.
Archaic use for a romantic partner. In modern Japanese, 恋人 or 愛人 are more common.
Living in a town where you know no one is lonely.
In old writings, there are many poems longing for one's lover.
Phrase composed of the verb 知る (to know) and 人 (person), literally 'a person one knows'. The archaic sense 'lover' derives from classical usage where knowing someone implied intimacy.