1660s, "not lit up," from un- (1) "not" + past participle of enlighten (v.). Meaning "not mentally illuminated" is attested from 1650s.
the devices by which unenlightened men preserved the unjust social order
lamentably unenlightened as to the laws
unemployment
unencumbered
unending
unendurable
un-English
unenlightened
unenthusiastic
unenviable
unequal
unequivocal
unerring