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