improper adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

c. 1300, propre, "adapted to some purpose, fit, apt; commendable, excellent" (sometimes ironic), from Old French propre "own, particular; exact, neat, fitting, appropriate" (11c.) and directly from Latin proprius "one's own, particular to itself," from pro privo "for the individual, in particular," from ablative of privus "one's own, individual" (see private (adj.)) + pro "for" (see pro-). Related: Properly; properness. As an adverb, "very exceedingly," from mid-15c., but since 19c. the use is considered vulgar.

From early 14c. as "belonging or pertaining to oneself; individual; intrinsic;" also as "pertaining to a person or thing in particular, special, specific; distinctive, characteristic;" also "what is by the rules, correct, appropriate, acceptable." From early 15c. as "separate, distinct; itself." Meaning "socially appropriate, decent, respectable" is recorded by 1704.

Proper name "name belonging to or relating to the person or thing in question, name given to an individual of a for distinction from others of the same class" is from c. 1300, a sense also preserved in astronomical proper motion "change in the apparent places of a celestial object in the sky relative to other stars or planets" (c. 1300). Proper noun is from mid-15c.

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.