In its Greek form, the prefix un- comes from the Greek alpha privative, which is a combining form that means "not." This prefix is used in Greek words such as ἄγνωστος (ágnostos), meaning "unknown" or "unaware."
In its Latin form, the prefix un- comes from the Latin prefix in- (or im-), which also means "not." This prefix is used in Latin words such as ingratus (meaning "ungrateful") and infamis (meaning "infamous").
In English, the prefix un- is used in both Greek- and Latin-derived words. For example, the word "unknown" is derived from the Greek prefix un-, while the word "ungrateful" is derived from the Latin prefix in-.