1. a. Being against established or accepted rules and standards: an illegitimate means of winning a debate.
b. Being against the law; illegal: an illegitimate contract.
c. Not valid or defensible: Their reasons for missing school are illegitimate.
d. Incorrectly deduced; illogical: an illegitimate conclusion.
e. Biology Unacceptable as a scientific name because of not conforming to the international rules of nomenclature.