Etymology
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will-o'-the-wisp (n.)

1660s, earlier Will with the wisp (c. 1600), from the masc. proper name Will + wisp "bundle of hay or straw used as a torch." Compare Jack-o'-lantern.

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O 

fifteenth letter of the alphabet, from a character that in Phoenician was called  'ain (literally "eye") and represented "a very peculiar and to us unpronounceable guttural" [Century Dictionary]. The Greeks also lacked the sound, so when they adopted the Phoenician letters they arbitrarily changed O's value to a vowel. (Thus there is no grounds for the belief that the form of the letter represents the shape of the mouth in pronouncing it.) The Greeks later added a special character for "long" O (omega), and the original became "little o" (omicron).

In Middle English and later colloquial use, o or o' can be an abbreviation of on or of, and is still literary in some words (o'clock, Jack-o'-lantern, tam-o'-shanter, cat-o'-nine-tails, will-o'-the-wisp, etc.).

O' the common prefix in Irish surnames is from Irish ó, ua (Old Irish au, ui) "descendant." 

The "connective" -o- is the usual connecting vowel in compounds taken or formed from Greek, where it often is the vowel in the stem. "[I]t is affixed, not only to terms of Greek origin, but also to those derived from Latin (Latin compounds of which would have been formed with the L. connecting or reduced thematic vowel, -i), especially when compounds are wanted with a sense that Latin composition, even if possible, would not warrant, but which would be authorized by the principles of Greek composition." [OED]

As "zero" in Arabic numerals it is attested from c. 1600, from the similarity of shape. Similarly the O blood type (1926) was originally "zero," denoting the absence of A and B agglutinogens.

As a gauge of track in model railroads, by 1905. For o as an interjection of fear, surprise, joy, etc., see oh.

The use of the colloquial or slang -o suffix in wino, ammo, combo, kiddo, the names of the Marx Brothers, etc., "is widespread in English-speaking countries but nowhere more so than in Australia" [OED].

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will (v.1)

Old English *willan, wyllan "to wish, desire; be willing; be used to; be about to" (past tense wolde), from Proto-Germanic *willjan (source also of Old Saxon willian, Old Norse vilja, Old Frisian willa, Dutch willen, Old High German wellan, German wollen, Gothic wiljan "to will, wish, desire," Gothic waljan "to choose").

The Germanic words are from PIE root *wel- (2) "to wish, will" (source also of Sanskrit vrnoti "chooses, prefers," varyah "to be chosen, eligible, excellent," varanam "choosing;" Avestan verenav- "to wish, will, choose;" Greek elpis "hope;" Latin volo, velle "to wish, will, desire;" Old Church Slavonic voljo, voliti "to will," veljo, veleti "to command;" Lithuanian velyti "to wish, favor," pa-velmi "I will," viliuos "I hope;" Welsh gwell "better").

Compare also Old English wel "well," literally "according to one's wish;" wela "well-being, riches." The use as a future auxiliary was already developing in Old English. The implication of intention or volition distinguishes it from shall, which expresses or implies obligation or necessity. Contracted forms, especially after pronouns, began to appear 16c., as in sheele for "she will." In early use often -ile to preserve pronunciation. The form with an apostrophe ('ll) is from 17c.

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will (n.)

Old English will, willa "mind, determination, purpose; desire, wish, request; joy, delight," from Proto-Germanic *wiljon- (source also of Old Saxon willio, Old Norse vili, Old Frisian willa, Dutch wil, Old High German willio, German Wille, Gothic wilja "will"), related to *willan "to wish" (see will (v.1)). The meaning "written document expressing a person's wishes about disposition of property after death" is first recorded late 14c.

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wisp (n.)

early 14c., "handful or bundle of hay, grass, etc.," used for burning or cleaning or as a cushion; perhaps from an unrecorded Old English word, cognate with Norwegian and Swedish visp "wisp," of unknown origin; sometimes said to be connected with whisk or with Middle Low German and Middle Dutch wispel "a measure of grain." Meaning "thin, filmy portion" first attested 1836.

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will (v.2)

Old English willian "to determine by act of choice," from will (n.). From mid-15c. as "dispose of by will or testament." Often difficult to distinguish from will (v.1).

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Jell-O (n.)

also sometimes Jello, trademark for powdered gelatin food, advertised from 1900 by Genesee Pure Food Co., Le Roy, N.Y.

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good will (n.)

Old English godes willan "state of wishing well to another;" see good (adj.) + will (n.). Meaning "cheerful acquiescence" is from c. 1300. In the commercial sense "degree of favor enjoyed through patronage of customers" from 1570s.

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*were-o- 

*wērə-o-, Proto-Indo-European root meaning "true, trustworthy."

It forms all or part of: aver; Varangian; veracious; veracity; verdict; veridical; verify; verisimilitude; verism; veritas; verity; very; voir dire; warlock.

It is the hypothetical source of/evidence for its existence is provided by: Latin verus "true;" Old Church Slavonic vera "faith," Russian viera "faith, belief;" Old English wær "a compact," Old Dutch, Old High German war, Dutch waar, German wahr "true;" Welsh gwyr, Old Irish fir "true."

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c/o 

addressing abbreviation for care of; common by 1889.

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