—adj. 1. born of parents who are not married to each other; born out of wedlock: an illegitimate child. 2. not legitimate; not sanctioned by law or custom.
3. unlawful; illegal: an illegitimate action. 4. irregular; not in good usage.
5. Logic.not in accordance with the principles of valid inference.
6. Obs.(formerly, in London)
a. of or pertaining to stage plays in which musical numbers were inserted because of laws that gave only a few theaters the exclusive right to produce straight dramas.
b. acting in or producing such productions.
—n. a person recognized or looked upon as illegitimate.