Etymology
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crack (n.)

"a split, an opening, narrow fracture," mid-15c., earlier "a splitting sound; a fart; the sound of a trumpet" (late 14c.), probably from crack (v.).

Meaning "sharp, resounding blow" is from 1836. Meaning "rock cocaine" is first attested 1985. The superstition that it is bad luck to step on sidewalk cracks has been traced to c. 1890. Meaning "try, attempt" first attested 1830, nautical, probably a hunting metaphor, from slang sense of "fire a gun."

At their head, apart from the rest, was a black bull, who appeared to be their leader; he came roaring along, his tail straight an [sic] end, and at times tossing up the earth with his horns. I never felt such a desire to have a crack at any thing in all my life. He drew nigh the place where I was standing; I raised my beautiful Betsey to my shoulder, took deliberate aim, blazed away, and he roared, and suddenly stopped. ["A Narrative of the Life of David Crockett, Written by Himself," Philadelphia, 1834]

Adjectival meaning "top-notch, superior, excellent, first rate" (as in a crack shot) is slang from 1793, perhaps from earlier verbal sense of "do any thing with quickness or smartness" [Johnson], or from the verb in the sense of "speak boastingly" and suggesting "having qualities to be proud of" [Century Dictionary]. Grose (1796) has "THE CRACK, or ALL THE CRACK. The fashionable theme, the go." To fall or slip through the cracks figuratively, "escape notice," is by 1975. Crack-brained "demented" is attested from 1630s. The biblical crack of doom is in reference to the sound (Old English translates it as swegdynna maest).

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cap (n.)

late Old English cæppe "hood, head-covering, cape," a general Germanic borrowing (compare Old Frisian and Middle Dutch kappe, Old High German chappa) from Late Latin cappa "a cape, hooded cloak" (source of Spanish capa, Old North French cape, French chape), a word of uncertain origin. Possibly a shortened from capitulare "headdress," from Latin caput "head" (from PIE root *kaput- "head").

The Late Latin word apparently originally meant "a woman's head-covering," but the sense was transferred to "hood of a cloak," then to "cloak" itself, though the various senses co-existed. Old English took in two forms of the Late Latin word, one meaning "head-covering," the other "ecclesiastical dress" (see cape (n.1)). In most Romance languages, a diminutive of Late Latin cappa has become the usual word for "head-covering" (such as French chapeau).

The meaning "soft, small, close-fitted head covering" in English is from early 13c., originally for women; extended to men late 14c.; extended to cap-like coverings on the ends of anything (as in hubcap) from mid-15c. The meaning "contraceptive device" is by 1916.

The meaning "cap-shaped piece of copper lined with gunpowder and used to ignite a firearm" is by 1825, hence cap-gun (1855); extended to paper strips used in toy pistols by 1872 (cap-pistol is from 1879).

Figurative thinking cap is from 1839 (considering cap is 1650s). Cap and bells (1781) was the insignia of a fool; cap and gown (1732) of a scholar. To set one's cap at or for (1773) means "use measures to gain the regard or affection of," usually in reference to a woman seeking a man's courtship.

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Quaker (n.)

"a member of the Christian denomination known as the Religious Society of Friends," 1651, said to have been applied to them in 1650 by Justice Bennett at Derby, from George Fox's admonition to his followers to "tremble at the Word of the Lord;" but the word was used earlier of foreign sects given to fits of shaking during religious fervor, and that is likely the source here. Either way, it never was an official name of the Religious Society of Friends.

The word in a literal sense of "one who or that which trembles" is attested from early 15c., an agent noun from quake (v.). The notion of "trembling" in religious awe is in Old English; quaking (n.) meaning "fear and reverence" especially in religion is attested from mid-14c.

There is not a word in the Scripture, to put David's condition into rime and meeter: sometimes he quaked and trembled, and lay roaring all the day long, that he watered his bed with his tears: and how can you sing these conditions (but dishonour the Lord) and say all your bones quake, your flesh trembled, and that you water your bed with your tears? when you live in pride and haughtiness, and pleasure, and wantonness .... ["A Brief Discovery of a threefold estate of Antichrist Now Extant in the world, etc.," an early Quaker work, London, 1653]

Figuratively, as an adjective, in reference to plain or drab colors (such as were worn by members of the sect) is by 1775. A Quaker gun (1809, American English), originally a log painted black and propped up to resemble the barrel of a cannon to deceive the enemy from a distance, is so called for the sect's noted pacifism. Philadelphia, Pennsylvania, has been known as the Quaker City at least since 1824. Related: Quakerish; Quakeress ("a female Quaker"); Quakerism; Quakerdom; Quakerly.

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