shoehorn (n.)

Old English horn "horn of an animal; projection, pinnacle," also "wind instrument" (originally one made from animal horns), from Proto-Germanic *hurni- (source also of German Horn, Dutch horen, Old Frisian horn, Gothic haurn), from PIE root *ker- (1) "horn; head."

Late 14c. as "one of the tips of the crescent moon." The name was retained for a class of musical instruments that developed from the hunting horn; the French horn is the true representative of the class. Of dilemmas from 1540s; of automobile warning signals from 1901. Slang meaning "erect penis" is suggested by c. 1600. Jazz slang sense of "trumpet" is by 1921. Meaning "telephone" is by 1945. Figurative senses of Latin cornu included "salient point, chief argument; wing, flank; power, courage, strength." Horn of plenty is from 1580s. To make horns at "hold up the fist with the two exterior fingers extended" as a gesture of insult is from c.1600.

Symbolic of cuckoldry since mid-15c. (the victim was fancied to grow one on his head). The image is widespread in Europe and perhaps as old as ancient Greece. The German linguist Hermann Dunger ('Hörner Aufsetzen' und 'Hahnrei', "Germania" 29, 1884) ascribes it to a custom surviving into 19c., "the old practice of engrafting the spurs of a castrated cock on the root of the excised comb, which caused them to grow like horns" [James Hastings, "Encyclopedia of Religion and Ethics"] but the image could have grown as well from a general gesture of contempt or insult made to wronged husbands, "who have been the subject of popular jest in all ages" [Hastings].

Middle English sho, "low-cut covering for the human foot," from Old English scoh, from Proto-Germanic *skokhaz (source also of Old Norse skor, Danish and Swedish sko, Old Frisian skoch, Old Saxon skoh, Middle Dutch scoe, Dutch schoen, Old High German scuoh, German Schuh, Gothic skoh). No known cognates outside Germanic, unless it somehow is connected with PIE root *skeu- "cover" (source also of second element in Latin ob-scurus).

The old plural form shoon lasted until 16c. The meaning "metal plate or rim nailed to the hoof of a horse or beast of burden to protect it from injury" is attested from c.1300. The distinction between shoe and boot (n.) is attested from c. 1400.

To stand in someone's shoes "see things from his or her point of view" is attested from 1767. Old shoe as a type of something worthless is attested from late 14c.

Shoes tied to the fender of a newlywed couple's car preserves the old custom (mentioned from 1540s) of throwing an old shoe at or after someone to wish them luck. Perhaps the association is with dirtiness, on the "muck is luck" principle.