Every American boy has been threatened with being "noogied" to tears unless he says "uncle." Why does my parent's brother have anything to do with mercy?
Of course he doesn't. Interestingly, saying or crying "uncle" is an Americanism, unknown in England except from recent contamination. The "uncle" who's married to your aunt is from Latin avunculus, meaning specifically your maternal uncle (that is, your mother's brother).(1) As so frequently happens, this is not the word for begging mercy at all. Instead, "uncle" is what's called a folk etymology, where an unfamiliar word's form and spelling come to resemble another, more familiar word. Amer. Speech 1976 LI. 281 says: " . . . the Irish original of the word is anacol (anacal, anacul) 'act of protecting; deliverance; mercy, quarter, safety,' a verbal noun from the Old Irish verb aingid 'protects' . . ." So, when you cry "uncle," you're actually crying for mercy. Is there any ultimate relationship with the hypothesized IE roots *ane- 'to breathe' and *aiw- 'vital force, life'?
"Uncle" brings to mind the expression to give (ask) quarter. The other words containing "quart-" all have something to do with four of something or a fourth of something. So a quart of milk is a fourth of a gallon; a quarter is a fourth of something. A quarterstaff may have been made by quartering a thick branch. All of this is faithful to the IE root, which is *kwetwer-.(2) By the 1300s, one could also speak of the quarter (or region) of a city, such as the slave quarter or the Jewish quarter. At some point, the area where you lived became your domicile, your "quarters." Particularly if you were a soldier, your were "quartered" somewhere, and the "quartermaster" was in charge of providing housing. So in thinking of begging "quarter," or freedom from being put to death, one possibility is the choice facing a defeated enemy of an ancient army: he either has to be killed or held for ransom. Of course, if he's held for ransom, he needs to be housed and fed, or "quartered," for an extended time while negotiations are carried out. Hence, he prays for "quarter."
But it may be more complicated than that. Unrelated to the "four" words, OE quartern (or cweartern) was a prison and is of obscure origin.(3) Hence, giving someone no quarter means to kill them rather than imprison them.
Yet another possibility is that this use grows out of an obsolete sense of "quarter," found in a couple of passages in Shakespeare, as meaning relations with someone, particularly fair dealing. So asking for quarter is just asking that they deal fairly with you.
1. Your father's brother was patruus. Being someone's patron is like being his uncle.
2. "Four" may also a descendant by way of a Germanic Great Consonent Shift. Another interesting descendent is carillon, which derives from a set of four bells. I also like cater-corner, which is the diagonally opposite, or "fourth" corner (folk etymology has given us "catty-corner" as a variant). A stone quarry is a four-sided excavation, while the quarry you hunt for is something you might capture by "boxing" it in, thus surrounding it on four sides. One kind of a quarrel is a four-sided arrow point, but the quarrel meaning "spat" is unrelated, deriving instead from a word meaning complain. Also unrelated is quartz, see SIREN.
some sort of corruption of L. carcer, prison (from which we get incarcerate);
derived from AS cweartern, wailing house (same root as quarrel=spat?) (hence
a concentration camp or prison).