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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goose (n.)

"a large waterfowl proverbially noted, I know not why, for foolishness" [Johnson], Old English gos "a goose," from Proto-Germanic *gans- "goose" (source also of Old Frisian gos, Old Norse gas, Old High German gans, German Gans "goose"), from PIE *ghans- (source also of Sanskrit hamsah (masc.), hansi (fem.), "goose, swan;" Greek khen; Latin anser; Polish gęś "goose;" Lithuanian žąsis "goose;" Old Irish geiss "swan"), probably imitative of its honking.

Geese are technically distinguished from swans and from ducks by the combination of feathered lores, reticulate tarsi, stout bill high at the base, and simple hind toe. [Century Dictionary]

Spanish ganso "goose" is from a Germanic source. Loss of "n" sound before "s" is normal in English (compare tooth). Plural form geese is an example of i-mutation. Meaning "simpleton, silly or foolish person" is from early 15c. To cook (one's) goose is attested by 1845, of unknown origin; attempts to connect it to Swedish history and Greek fables are unconvincing. Goose-egg "zero" is attested by 1866 in baseball slang, from being large and round. The goose that lays golden eggs (15c.) is from Aesop.

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positive (adj.)

early 14c., originally a legal term meaning "formally laid down, decreed or legislated by authority" (opposed to natural),  from Old French positif (13c.) and directly from Latin positivus "settled by agreement, positive" (opposed to naturalis "natural"), from positus, past participle of ponere "put, place" (see position (n.)).

The sense of "absolute" is from mid-15c. Meaning in philosophy of "dealing only with facts" is from 1590s. Sense broadened to "expressed without qualification" (1590s), then, of persons, "confident in opinion" (1660s). The meaning "possessing definite characters of its own" is by 1610s. The mathematical use for "greater than zero" is by 1704. Psychological sense of "concentrating on what is constructive and good" is recorded from 1916. Positive thinking is attested from 1953. The sense in electricity is from 1755.  

There are probably no two bodies differing in nature which are not capable of exhibiting electrical phaenomena, either by contact, pressure, or friction ; but the first substances in which the property was observed, were vitreous and resinous bodies ; and hence the different states were called states of resinous and vitreous electricity ; and resinous bodies bear the same relation to flint glass, as silk. The terms, negative and positive electricity, have been likewise adopted, on the idea, that the phaenomena depend upon a peculiar subtile fluid, which becomes in excess in the vitreous, and deficient in the resinous bodies ; and which is conceived by its motion and transfer, to produce the electrical phaenomena. [Sir Humphry Davy, "Elements of Chemical Philosophy," London, 1812]
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