Etymology
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William 

masc. proper name, from Old North French Willaume, Norman form of French Guillaume, of Germanic origin (cognates: Old High German Willahelm, German Wilhelm), from willio "will" (see will (n.)) + helma "helmet," from Proto-Germanic *helmaz "protective covering" (from PIE root *kel- (1) "to cover, conceal, save;" compare helm (n.2)). After the Conquest, the most popular given name in England until supplanted by John.

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Wilhelm 
German form of William (q.v.). Fem. form is Wilhelmina. Wilhelmine (adj.) is "pertaining to the reign of Wilhelm II," emperor of Germany 1888-1918. Berlin's Wilhelmstrasse was the pre-1945 headquarters of the German foreign office, hence used metonymically for "German foreign policy" (compare Quai d'Orsay).
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billy (n.)
"club," 1848, American English, originally burglars' slang for "crowbar;" meaning "policeman's club" first recorded 1856, probably from nickname of William, applied to various objects (compare jack, jimmy, jenny). But compare French bille "a short, stout stick" (see billet (n.1)). Billy-goat as a familiar name for a male goat is from 1826.
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hillbilly (n.)

"southern Appalachian person," by 1892, from hill (n.) + Billy/Billie, popular or pet form of William. In reference to a type of U.S. folk music, first attested 1924.

I would hate to see some old railroad man come here and take my job, and then, I don't think it is right to hire some Hill Billy and give him the same right as I just because he was hired the same time I was. [The Railroad Trainmen's Journal, vol. ix, July 1892]
In short, a Hill-Billie is a free and untrammelled white citizen of Alabama, who lives in the hills, has no means to speak of, dresses as he can, talks as he pleases, drinks whiskey when he gets it, and fires of his revolver as the fancy takes him. [New York Journal, April 23, 1900]

In Scott's collection of Border ballads, billie is a frequent term of address or intimacy, "comrade, companion, a brother in arms," "a term expressive of affection and familiarity" also "a brother; a wooer of a woman," and generally "a young man" [Jamieson, 2nd edition]. It is said to be a variant of bully (n.) in its old sense of  "sweetheart," also "fine fellow."

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*kel- (1)

Proto-Indo-European root meaning "to cover, conceal, save."

It forms all or part of: Anselm; apocalypse; Brussels; caliology; Calypso; calyx; ceiling; cell; cellar; cellular; cellulite; cellulitis; cilia; clandestine; cojones; coleoptera; color; conceal; eucalyptus; hall; hell; helm (n.2) "a helmet;" helmet; hold (n.2) "space in a ship below the lower deck;" hole; hollow; holster; housing (n.2) "ornamental covering;" hull (n.1) "seed covering;" kil-; kleptomania; occult; rathskeller; supercilious; Valhalla; William.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit cala "hut, house, hall;" Greek kalia "hut, nest," kalyptein "to cover," koleon, koleos "sheath," kelyphos "shell, husk;" Latin cella "small room, store room, hut," celare "to hide, conceal," clam "secret," clepere "to steal, listen secretly to;" Old Irish cuile "cellar," celim "hide," Middle Irish cul "defense, shelter;" Gothic hulistr "covering," Old English heolstor "lurking-hole, cave, covering," Gothic huljan "to cover over," hulundi "hole," hilms "helmet," halja "hell," Old English hol "cave," holu "husk, pod;" Old Prussian au-klipts "hidden;" Old Church Slavonic poklopu "cover, wrapping."

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Nobodaddy (n.)

c. 1793, William Blake's derisive name for the anthropomorphic God of Christianity. The name reflects nobody + daddy.

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Occam's razor (n.)

when two competing hypotheses explain the data equally well, choose the simpler. Or, as Sir William Hamilton puts it, "Neither more, nor more onerous, causes are to be assumed, than are necessary to account for the phenomena." Named for English philosopher William of Ockham or Occam (c. 1285-c. 1349), "The Invincible Doctor," who expressed it with Entia non sunt multiplicanda praeter neccssitatem.

So called after William of Occam (died about 1349): but, as a historical fact, Occam does not make much use of this principle, which belongs rather to the contemporary nominalist William Durand de St. Pourçain (died 1332). [Century Dictionary]
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McGuffey's 
children's reader, first published 1836, created by Ohio educator and linguist William Holmes McGuffey (1800-1873).
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weltanschauung (n.)
1868 (William James), from German Weltanschauung, from welt "world" (see world) + anschauung "perception" (related to English show).
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Congreve 

in reference to solid-fuel rockets (1809) or matches (1839), a reference to English inventor Sir William Congreve (1772-1828), who learned the tactic of rocket warfare while serving in India. He was a descendant of the family of William Congreve the Restoration playwright (1670-1729), being probably his great-great-nephew.

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