Etymology
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improper (adj.)

mid-15c., "not true," from Old French impropre (14c.) and directly from Latin improprius "not proper," from assimilated form of in- "not, opposite of" (see in- (1)) + proprius (see proper). Meaning "not suited, unfit" is from 1560s; that of "not in accordance with good manners, modesty, or decency" is from 1739. Related: Improperly (late 14c.).

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Definitions of improper

improper (adj.)
not suitable or right or appropriate;
improper attire for the golf course
slightly improper to dine alone with a married man
improper medication
improper (adj.)
not conforming to legality, moral law, or social convention;
improper banking practices
improper (adj.)
not appropriate for a purpose or occasion;
Synonyms: wrong
From wordnet.princeton.edu