back-breaking adj.

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.

Old English brecan "to divide solid matter violently into parts or fragments; to injure, violate (a promise, etc.), destroy, curtail; to break into, rush into; to burst forth, spring out; to subdue, tame" (class IV strong verb; past tense bræc, past participle brocen), from Proto-Germanic *brekanan (source also of Old Frisian breka, Dutch breken, Old High German brehhan, German brechen, Gothic brikan), from PIE root *bhreg- "to break."

Closely related to breach (n.), brake (n.1), brick (n.). The old past tense brake is obsolete or archaic; the past participle is broken, but shortened form broke is attested from 14c. and was "exceedingly common" [OED] 17c.-18c.

Of bones in Old English. Formerly also of cloth, paper, etc. The meaning "escape by breaking an enclosure" is from late 14c. The intransitive sense of "be or become separated into fragments or parts under action of some force" is from late 12c. The meaning "lessen, impair" is from late 15c. That of "make a first and partial disclosure" is from early 13c. The sense of "destroy continuity or completeness" in any way is from 1741. Of coins or bills, "to convert to smaller units of currency," by 1882.

In reference to the heart from early 13c. (intransitive); to break (someone's) heart is late 14c. Break bread "share food" (with) is from late 14c. To break ground is from 1670s as "to dig, plow," from 1709 in the figurative sense of "begin to execute a plan." To break the ice "overcome the feeling of restraint in a new acquaintanceship" is from c. 1600, in reference to the "coldness" of encounters of strangers. Break wind is attested from 1550s. To break (something) out (1890s) probably is an image from dock work, of freeing cargo before unloading it.

The ironic theatrical good luck formula break a leg (by 1948, said to be from at least 1920s) has parallels in German Hals- und Beinbruch "break your neck and leg," and Italian in bocca al lupo. Evidence of a highly superstitious craft (see Macbeth). According to Farmer & Henley, in 17c. the expression was used euphemistically, of a woman, "to have a bastard."