adjective

1. born of parents not married to each other; bastard
2. incorrectly deduced: not logical an illegitimate conclusion
3. contrary to law or rules; illegal; unlawful
4. not in keeping with accepted usage: said of words or phrases

Etymology: < ML illegitimatus, pp. of illegitimare, to make illegitimate < L illegitimus, not lawful: see in- & legitimate