From un- + proper.

• (British) IPA: /ʌnˈpɹɒpə/

unproper

1. (obsolete) Improper, not according with fact or reason; wrong, irregular. [14th]
   • c. 1631, John Donne, letter to Sir Henry Goodyere, Works, Letter XC, p. 409:

     Sir, as I said last time, labour to keep your alacrity and dignity, in an even temper: for in a dark sadness, indifferent things seem abominable, or necessary, being neither; as trees, and sheep, to melancholy night-walkers, have unproper shapes.

2. (now rare) Improper, not suited for its use or application; inappropriate. [from 16th c.]
   • 1627 (indicated as 1626), Francis [Bacon], “VII. Century.”, in Sylua Syluarum: Or A Naturall Historie. In Ten Centuries. […], London: […] William Rawley […]; [p]rinted by J[ohn] H[aviland] for William Lee […], →OCLC ↗, page 694 ↗:

     The pleasure in the act of Venus is the greatest of the pleasures of the senses: the matching of it with itch is unproper; though that also be pleasing to the touch.

   • 2009, "Own Goal for Football", The Times, 10 Oct 09:

     Transparency is paramount. If football’s guardians cannot deliver it they will, rightly, be deemed to be every bit as unfit and unproper to play a role in administering the sport as any secretive investor they may feel motivated to investigate.

3. (obsolete) Not belonging to a given person; someone else's. [17th c.]
   • c. 1603–1604 (date written), William Shakespeare, “The Tragedie of Othello, the Moore of Venice”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act IV, scene i]:

     There's Millions now aliue, That nightly lye in those vnproper beds, Which they dare sweare peculiar.

4. (rare) Improper, not according with good standards of behaviour; indecent, indecorous. [from 19th c.]