late 14c., "lacking wisdom or knowledge; unaware," from Old French ignorant (14c.), from Latin ignorantem (nominative ignorans) "not knowing, ignorant," present participle of ignorare "not to know, to be unacquainted; mistake, misunderstand; take no notice of, pay no attention to," from assimilated form of in- "not, opposite of" (see in- (1)) + Old Latin gnarus "aware, acquainted with" (source also of Classical Latin noscere "to know," notus "known"), from Proto-Latin suffixed form *gno-ro-, suffixed form of PIE root *gno- "to know." Also see uncouth.
Form influenced by related Latin ignotus "unknown, strange, unrecognized, unfamiliar." Colloquial sense of "ill-mannered, uncouth, knowing nothing of good manners" attested by 1886. As a noun, "ignorant person," from mid-15c. Related: Ignorantly.