Synonyms for Improper:

adj.

all (adjective)
wrong.
disrespectful (adjective)
impertinent, insolent, irreverent, impudent, disrespectful.
improper (adjective)
unseemly, indecorous, untoward, unfit, indelicate, indecent, unsuitable, inappropriate.
inappropriate (adjective)
out, of place, inapt, inappropriate, unsuitable, unbecoming, ill-matched, unfit, unsuited, inapposite, unbefitting, malapropos.
irregular (adjective)
irregular, quirky.
nonconforming (adjective)
fey, inconsistent, irregular, eccentric, odd, curious, quirky, freakish, abnormal, extraordinary, unusual, crazy, crosspatch, aberrant, incongruous, oddity, un-ordinary, unconventional, incompatible, mismatch, oddball, out of the ordinary, deviant, strange, nonconforming, bizarre, anomalous, uncommon, unfamiliar, outlandish, unique, divergent, exceptional, untypical, singular, uncustomary, different, idiosyncratic, misfit, erratic, peculiar, novel, deranged, off, funny, atypical.
not suitable (adjective)
inapposite, inapplicable, awkward, preposterous, inexpedient, incongruous, unsuited, inharmonious, erroneous, incorrect, uncomely, unfit, inaccurate, ludicrous, inapt, ill-advised, irregular, unbefitting, malapropos, inadvisable, abnormal, inappropriate, inopportune, unwarranted, wrong, false, Unapt, discordant, odd, discrepant, imprudent, unsuitable, undue.
vulgar, immoral (adjective)
naughty, unconventional, indecorous, indelicate, unethical, suggestive, untoward, lewd, unseemly, indecent, unbecoming.
wicked (adjective)
evildoing, debauched, lawbreaking, black, base, indecent, wrongdoing, iniquitous, sinful, reprehensible, wicked, corrupt, damnable, criminal, debased, Misdoing, villainous, sadistic, perverse, shocking, bad, immoral, depraved, degenerate, blameworthy, sinister, felonious, scandalous, obnoxious.
wrong (adjective)
flawed, fallacious, false, erroneous, untrue, faulty, mistaken, bad, inaccurate, incorrect, wrong, illegal, aberrant.

n.

fraudulent, unauthorized, unconstitutional, shady, clandestine. intimate, sensual, amorous, animal, usual, obscene, Unapt, inept, vulgar, agree, romantic. gross, amoral, immodest, sordid, evil, perverted, unethical, naughty. inconvenient. immoral (noun)
lewd, smutty, suggestive, naughty, immodest.
improper (noun)
unconventional, untoward, unbecoming, irregular, inappropriate, indecent, unseemly, unsuitable, incorrect, wrong, unlawful, indecorous, uncomely, out-of-the-way.
unsuitable (noun)
inapplicable, inexpedient, inharmonious, awkward, preposterous, discordant, discrepant, unwarranted, inadvisable, imprudent, ill-advised, inopportune, ludicrous, undue, Unapt.

Other synonyms:

inconvenient, inopportune. inept. naughty. Unapt. forward
immodest.
unlawful
unlawful.
unwise
inadvisable.
Other relevant words:
inconvenient, suggestive, inept, preposterous, out-of-the-way, gross, inharmonious, undue, discrepant, uncomely, amoral, naughty, discordant, sordid, clandestine, unauthorized, inapplicable, ludicrous, ill-advised, evil, immodest, vulgar, inadvisable, unlawful, unethical, awkward, perverted, sensual, lewd, inexpedient, obscene, inopportune, Unapt, imprudent, smutty, unwarranted.

Usage examples for improper

  1. " You take the money on these conditions: it is almost as improper of you to wish to evade them, as it would be of me to assist you. – Put Yourself in His Place by Charles Reade
  2. Had she done anything improper on this her first entry into the world? – Castle Richmond by Anthony Trollope
  3. She wouldn't see them, and then I shouldn't like to show her the improper ones. – The Dark Tower by Phyllis Bottome