1610s, "quality or fact of being improper," from French impropriété (16c.) or directly from Latin improprietas "impropriety," from improprius "improper" (see improper). As "improper thing," 1670s.
Entries linking to impropriety
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.).