Synonyms for Illegal:

adj.

contraband (adjective)
bootlegged.
criminal (adjective)
dirty, wrong.
illegal (adjective)
taboo, punishable, forbidden, not legal, prohibited, verboten, felonious, contraband, smuggled, actionable, offensive, lawless, not approved, proscribed, unconstitutional, outlawed, extralegal, outside the law, unauthorized, illicit, unlawful, unwarrantable, bootleg, wrongful, banned, unlicensed, illegitimate, Interdicted, irregular, unjust, unwarranted, criminal.
illegitimate (adjective)
improper, misbegotten.
misbegotten (adjective)
stolen.
off-limits (adjective)
out of bounds, against the law, outlawed.
prohibited (adjective)
not approved.
undue (adjective)
unwarrantable.
wrong (adjective)
false, flawed, bad, erroneous, improper, faulty, aberrant, mistaken, wrong, fallacious, inaccurate, untrue, incorrect.

n.

exile, expatriate, emigre, immigration, asylum, immigrant, migrant, emigrant, residence, refugee. no-go, law, out of bounds, in contravention of something. illegal (noun)
bootleg, outlaw, illegitimate, unratified, dirty, hot, ill-gotten, nonlegal, black, illicit, ineligible, mislabeled, felonious, misbranded, penal, under-the-counter, extrajudicial, prohibited, embezzled, extralegal, unlawful, punishable, criminal, amerciable, contraband, outlawed, banned, smuggled, misappropriated.
illicit (noun)
uncertified, after hours, unwarrantable, bootlegged, proscribed, not approved, irregular, black market, not legal, unwarranted, misbegotten, outside the law, against the law, Interdicted, stolen.

Other synonyms:

no-go. baseborn
misbegotten.
Other relevant words:
ineligible, misbranded, nonlegal, misappropriated, proscribed, banned, unratified, extrajudicial, outlawed, misbegotten, not legal, unwarranted, contraband, smuggled, ill-gotten, unwarrantable, against the law, outlaw, Interdicted, black market, under-the-counter, not approved, extralegal, embezzled, penal, no-go, black, irregular, prohibited, outside the law, amerciable, mislabeled, dirty, hot.

Usage examples for illegal

  1. " There is much reason to suspect your vessel of illegal practices; and, in the name of the Queen, I demand access to her papers, and the liberty of a free search into her cargo and crew. – The Water-Witch or, The Skimmer of the Seas by James Fenimore Cooper
  2. The death of Lanfranc, who retained great influence over him, gave soon after a full career to his tyranny; and all orders of men found reason to complain of an arbitrary and illegal administration. – The History of England, Volume I by David Hume
  3. When you read it you realized that you were committing an illegal action? – The Gadfly by E. L. Voynich