not proper; not strictly belonging, applicable, correct, etc.; erroneous: He drew improper conclusions from the scant evidence.
not in accordance with propriety of behavior, manners, etc.: improper conduct at a funeral.
unsuitable or inappropriate, as for the purpose or occasion: improper attire for a formal dance.
abnormal or irregular: improper functioning of the speech mechanism.
Origin: 1535–45; < Latinimproprius. See im-2, proper
Synonyms 1–3. inapplicable, unsuited, unfit. 2. indecorous. Improper, indecent, unbecoming, unseemly are applied to that which is unfitting or not in accordance with propriety. Improper has a wide range, being applied to whatever is not suitable or fitting, and often specifically to what does not conform to the standards of conventional morality: improper diet; improper behavior in church; improper language.Indecent a strong word, is applied to what is offensively contrary to standards of propriety and especially of modesty: indecent behavior, literature.Unbecoming is applied to what is especially unfitting in the person concerned: conduct unbecoming a minister.Unseemly is applied to whatever is unfitting or improper under the circumstances: unseemly mirth.
1531 (implied in improperly) "not true," from Fr. impropre (14c.), from L. improprius, from in "not" + proprius (see proper). Meaning "not suited, unfit" is from 1570; that of "not in accordance with good manners, modesty, decency" is from 1739.