backscratcher (n.)

Old English bæc "back," from Proto-Germanic *bakam (cognates: Old Saxon and Middle Dutch bak, Old Frisian bek), with no known connections outside Germanic. In other modern Germanic languages the cognates mostly have been ousted in this sense by words akin to Modern English ridge (such as Danish ryg, German Rücken).

Many Indo-European languages show signs of once having distinguished the horizontal back of an animal (or a mountain range) from the upright back of a human. In other cases, a modern word for "back" may come from a word related to "spine" (Italian schiena, Russian spina) or "shoulder, shoulder blade" (Spanish espalda, Polish plecy).

By synecdoche, "the whole body," especially with reference to clothing. The meaning "upright part of a chair" is from 1520s. As a U.S. football position by 1876, so called from being behind the line of rushers; further distinguished according to relative position as quarterback, halfback, fullback. To turn (one's) back on (someone or something) "ignore" is from early 14c.

To know (something) like the back of one's hand, implying familiarity, is first attested 1893 in a dismissive speech made to a character in Robert Louis Stevenson's "Catriona":

If I durst speak to herself, you may be certain I would never dream of trusting it to you; because I know you like the back of my hand, and all your blustering talk is that much wind to me.

The story, a sequel to "Kidnapped," has a Scottish setting and context, and the back of my hand to you was noted in the late 19th century as a Scottish expression meaning "I will have nothing to do with you" [see Longmuir's edition of Jamieson's Scottish dictionary]. In English generally, the back of (one's) hand has been used to imply contempt and rejection at least since 1300. Perhaps the connection of a menacing dismissal is what made Stevenson choose that particular anatomical reference.

c. 1400, scracchen, transitive, "mark or wound slightly on the surface by a scraping or tearing action with something rough, sharp, or pointed," probably a fusion of Middle English scratten and crachen, both meaning "to scratch," both of uncertain origin. Also compare scr-. Related: Scratched; scratching.

The meaning "relieve skin irritation by a scraping motion with the nails or claws or a scratcher" is by 1520s. The billiards sense of "hit the cue ball into a pocket" is recorded by 1909 (also, originally, itch), though earlier it meant "a lucky shot" (1850). The meaning "to withdraw (a horse) from a race" is 1865, from notion of scratching its name off a list of competitors; the phrase was used in a non-sporting sense of "cancel a plan, etc." by 1680s.

To scratch the surface "make only slight progress in penetrating or understanding" is from 1882. To scratch (one's) head as a gesture of perplexity is recorded from 1712. The plastering scratch-coat, roughened by scratching before it sets, is by 1891.