1866, originally in reference to surfaces such as shell casings of beetle wings, from French aéroplane (1855), from Greek-derived aero- "air" (see air (n.1)) + stem of French planer "to soar," from Latin planus "level, flat" (from PIE root *pele- (2) "flat; to spread").
The word was later extended to the wing of a heavier-than-air flying machine. The use of the word in reference to the machine itself is first attested 1873 and probably is an independent coinage in English. Also see airplane. Ancient Greek had a word aeroplanos, but it meant "wandering in the air," from planos "wandering" (see planet).
<a href="https://www.etymonline.com/word/monoplane">Etymology of monoplane by etymonline</a>
Harper, D. (n.d.). Etymology of monoplane. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/monoplane