Etymology
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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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egomania (n.)

"obsessive self-centeredness," 1825 (in a letter of English critic William Sidney Walker, published in 1852), from ego + mania. Not in common use before 1890s, where it translated German Ich-Sucht.

[The egomaniac, as opposed to the megalomaniac,] does not regard it as necessary to dream of himself as occupying some invented social position. He does not require the world or its appreciation to justify in his own eyes himself as the sole object of his own interest. He does not see the world at all. Other people simply do not exist for him. The whole 'non-Ego' appears in his consciousness merely as a vague shadow or a thin cloud. The idea does not even occur to him that he is something out of the common, that he is superior to other people, and for this reason either admired or hated ; he is alone in the world ; more than that, he alone is the world and everything else, men, animals, things are unimportant accessories, not worth thinking about. [Max Nordau, "Degeneration," English translation, 1895]

Nordau's book was much-read, debated, and cited at the time and the word was associated with him (e.g. The Agora, July 1895).

Walker's use aside, its infrequent print appearance before 1895 seems to have been largely in the side of medicine that dealt with psychological matters:

The most frequent, yet the most extraordinary of these perversions of temper, are seen in young females. It is a species of aberration of the intellect, but short of insanity, real enough, but exaggerated, fictitious, factitious, and real at the same time. It frequently has its origin in dyspepsia, hysteria, or other malady, and in emotion of various kinds, such as disappointment, vexation, &c. Its object is frequently to excite and to maintain a state of active sympathy and attention, for which there, is as it were, a perpetual, morbid, and jealous thirst. It was rather aptly designated, by the clever relative of one patient, an ego-mania. [Marshall Hall, M.D., "Practical Observations and Suggestions in Medicine," London, 1845]
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humor (n.)

mid-14c., "fluid or juice of an animal or plant," from Old North French humour "liquid, dampness; (medical) humor" (Old French humor, umor; Modern French humeur), from Latin umor "body fluid" (also humor, by false association with humus "earth"); related to umere "be wet, moist," and to uvescere "become wet" (see humid).

In old medicine, "any of the four body fluids" (blood, phlegm, choler, and melancholy or black bile).

The human body had four humors—blood, phlegm, yellow bile, and black bile—which, in turn, were associated with particular organs. Blood came from the heart, phlegm from the brain, yellow bile from the liver, and black bile from the spleen. Galen and Avicenna attributed certain elemental qualities to each humor. Blood was hot and moist, like air; phlegm was cold and moist, like water; yellow bile was hot and dry, like fire; and black bile was cold and dry, like earth. In effect, the human body was a microcosm of the larger world. [Robert S. Gottfried, "The Black Death," 1983]

 Their relative proportions were thought to determine physical condition and state of mind. This gave humor an extended sense of "mood, temporary state of mind" (recorded from 1520s); the sense of "amusing quality, funniness, jocular turn of mind" is first recorded 1680s, probably via sense of "whim, caprice" as determined by state of mind (1560s), which also produced the verb sense of "indulge (someone's) fancy or disposition." Modern French has them as doublets: humeur "disposition, mood, whim;" humour "humor." "The pronunciation of the initial h is only of recent date, and is sometimes omitted ..." [OED].

For aid in distinguishing the various devices that tend to be grouped under "humor," this guide, from Henry W. Fowler ["Modern English Usage," 1926] may be of use:

HUMOR: motive/aim: discovery; province: human nature; method/means: observation; audience: the sympathetic
WIT: motive/aim: throwing light; province: words & ideas; method/means: surprise; audience: the intelligent
SATIRE: motive/aim: amendment; province: morals & manners; method/means: accentuation; audience: the self-satisfied
SARCASM: motive/aim: inflicting pain; province: faults & foibles; method/means: inversion; audience: victim & bystander
INVECTIVE: motive/aim: discredit; province: misconduct; method/means: direct statement; audience: the public
IRONY: motive/aim: exclusiveness; province: statement of facts; method/means: mystification; audience: an inner circle
CYNICISM: motive/aim: self-justification; province: morals; method/means: exposure of nakedness; audience: the respectable
SARDONIC: motive/aim: self-relief; province: adversity; method/means: pessimism; audience: the self
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