mid-15c., "not true," from French impropre (14c.), from Latin improprius, 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, decency" is from 1739. Related: Improperly (late 14c.).