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
  1. adjective
    contrary to or forbidden by law
    “an illegitimate seizure of power”
    synonyms: illicit, outlaw, outlawed, unlawful
    prohibited by law or by official or accepted rules
  2. adjective
    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
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    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
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  3. noun
    the illegitimate offspring of unmarried parents
    synonyms: bastard, by-blow, illegitimate child, love child, whoreson
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    type of:
    issue, offspring, progeny
    the immediate descendants of a person
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