Improper

google

ref

late Middle English: from French impropre or Latin improprius, from in- ‘not’ + proprius ‘one's own, proper’.

wiktionary

ref

From Middle French impropre, from Latin improprius(“not proper”), from in- + proprius(“proper”).

etymonline

ref

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.).