adj not according to law; not regular or authorized; unlawful; improper.
2. illegitimate ·adj unlawfully begotten; born out of wedlock; bastard; as, an illegitimate child.
3. illegitimate ·adj not authorized by good usage; not genuine; spurious; as, an illegitimate word.
4. illegitimate ·adj not legitimately deduced or inferred; illogical; as, an illegitimate inference.
5. illegitimate ·vt to render illegitimate; to declare or prove to be born out of wedlock; to bastardize; to illegitimatize.