backpack (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.

early 13c., pak, pake, "a bundle or package (of cloth, merchandise, etc.)," also "a bag or purse for carrying things," probably from a Low German word (compare Middle Dutch pac, pack "bundle," Middle Low German pak, Middle Flemish pac, attested from late 12c.) and taken into English from the wool traders in Flanders; or possibly from Old Norse pakki. All are of unknown origin. Italian pacco is a Dutch loan word; French pacque probably is from Flemish.

Especially a bundle enclosed in a wrapping and bound fast with cords. Meaning "set of persons" (usually of a low character) is from late 14c. and is older than sense of "group of instinctively herding hunting animals" (mid-15c.). Extended to "complete set of playing cards" (1590s), floating ice (1791), bundled cigarettes (1865), and submarines (1943).

Meaning "knapsack on a frame" is attested from 1916. Pack of lies is attested from 1763. Meaning "a person of low character" (usually with naughty) is by 1520s.