Something illegitimate is unlawful, or improper. If you're a doctor, even a great doctor, but you were never certified by the medical board, then you're an illegitimate doctor.

Illegitimate has the root for "law" in it, and something illegitimate is outside the law or custom. A child not born to married parents is known as an illegitimate child, although so many people are making different choices about how to raise families, that the stigma is dropping away, especially if the child isn't the result of philandering (cheating). In King Lear, Edmund is mad that he's an illegitimate son, because it means he can't inherit the throne.

Definitions of illegitimate

adj contrary to or forbidden by law

“an illegitimate seizure of power”
illicit, outlaw, outlawed, unlawful
prohibited by law or by official or accepted rules

adj of marriages and offspring; not recognized as lawful

unauthorised, unauthorized
not endowed with authority
prohibited by law or by official or accepted rules
conceived in adultery
base, baseborn
bastardly, misbegot, misbegotten, spurious
born out of wedlock
not having a known or legally responsible father
(of marriages) illicit or informal
unlawful, wrongful
having no legally established claim
having no cogency or legal force
of marriages and offspring; recognized as lawful
authorised, authorized
endowed with authority
established by or founded upon law or official or accepted rules
born in wedlock; enjoying full filial rights
left-handed, morganatic
(of marriages) of a marriage between one of royal or noble birth and one of lower rank; valid but with the understanding that the rank of the inferior remains unchanged and offspring do not succeed to titles or property of the superior
lawful, rightful, true
having a legally established claim
well grounded in logic or truth or having legal force
show more antonyms...

n the illegitimate offspring of unmarried parents

bastard, by-blow, illegitimate child, love child, whoreson
Type of:
issue, offspring, progeny
the immediate descendants of a person

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