The Oxford Thesaurus An A-Z Dictionary of Synonyms - illegitimateadj.

1) See illegal. 2) bastard, natural, fatherless, born out of wedlock, born on the wrong side of the blanket, misbegotten: Illegitimate children were often offered for adoption in those days. 3) irregular, improper, incorrect, non-standard, invalid, unauthorized, spurious: Purists consider the reason is because an illegitimate usage.