Etymology
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synecdoche (n.)
"figure of speech in which a part is taken for the whole or vice versa," late 15c. correction of synodoches (late 14c.), from Medieval Latin synodoche, alteration of Late Latin synecdoche, from Greek synekdokhe "the putting of a whole for a part; an understanding one with another," literally "a receiving together or jointly," from synekdekhesthai "supply a thought or word; take with something else, join in receiving," from syn- "with" (see syn-) + ek "out" (see ex-) + dekhesthai "to receive," related to dokein "seem good" (from PIE root *dek- "to take, accept"). Typically an attribute or adjunct substituted for the thing meant ("head" for "cattle," "hands" for "workmen," "wheels" for "automobile," etc.). Compare metonymy. Related: Synecdochical.
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*dek- 
Proto-Indo-European root meaning "to take, accept."

It forms all or part of: condign; dainty; decent; decor; decorate; decorous; deign; dignify; dignity; diplodocus; disciple; discipline; disdain; docent; Docetism; docile; docimacy; doctor; doctrine; document; dogma; dogmatic; doxology; heterodox; indignance; indignant; indignation; indignity; orthodox; paradox; synecdoche.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit daśasyati "shows honor, is gracious," dacati "makes offerings, bestows;" Greek dokein "to appear, seem, think," dekhesthai "to accept;" Latin decere "to be fitting or suitable," docere "to teach," decus "grace, ornament."
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boot camp (n.)
"training station for recruits," by 1941, U.S. Marines slang, said to be from boot (n.1) as slang for "recruit," which is attested by 1915 and supposedly dates from the Spanish-American War and is a synecdoche from boots "leggings worn by U.S. sailors."
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merism (n.)

1894 in the biological sense "repetition of parts in living things;" earlier in rhetoric, "synecdoche in which totality is expressed by contrasting parts" (such as high and low, young and old); from Modern Latin merismus, from Greek merismos "a dividing, division, a partition," from merizein "to divide," from meros "a part, a share" (from PIE root *(s)mer- (2) "to get a share of something"). Related: Meristic. Merismatic "dividing by the formation of internal partitions" is attested by 1849.

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shaft (n.1)

"long, slender rod," originally "staff or pole forming the body of a spear or lance; spear-shaft," also, perhaps by synecdoche, "spear;" Middle English shafte, from Old English sceaft from Proto-Germanic *skaftaz (source also of Old Norse skapt, Old Saxon skaft, Old High German scaft, German schaft, Dutch schacht, not found in Gothic).

OED suggests this might be explained as a Germanic passive past participle of PIE root *(s)kep- "to cut, to scrape" (source of Old English scafan "to shave, scrape, polish") on notion of "tree branch stripped of its bark." But compare Latin scapus "shaft, stem, shank," Greek skeptron "a staff" (see scepter) which appear to be cognates.

Extended generally to any body of long, cylindrical shape; the meaning "beam or ray" (of light, etc.) is attested from c. 1300; that of "arrow" (especially a long one, used with a long bow) is from c. 1400; that of "a long, straight handle of a tool or utensil" from 1520s. The mechanical sense "long rotating rod for transmission of motive power in a machine" is from 1680s.

The vulgar slang meaning "penis" is recorded by 1719 on notion of "columnar part" (late 14c.); hence probably modern slang shaft (v.) and the related noun meaning "act of unfair treatment" (1959), though some early sources insist this is from the notion of a wound.

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back (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.

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