Illegitimate means "illegal," and legitimate means the precise opposite,"legal" (See illegal). Illegitimate also has these specialized meanings: (1) "born out of wedlock (born a bastard)"; (2) "irregular" (an illegitimate forward pass); (3) "invalid" (a logically illegitimate conclusion). Legitimate is the antonym of illegitimate in each of these three meanings and is also used to mean "rightful" (a legitimate sovereign); "genuine" (a legitimate claim); and "pertaining to professionally produced stage plays" (the legitimate theater), as opposed to radio, television, films, and other forms of entertainment.
Dictionary of problem words and expressions. Harry Shaw. 1975.