Etymology
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combine (v.)

early 15c., "to associate, unite, join two or more things together" (transitive), from Old French combiner (14c.) and directly from Late Latin combinare "to unite, yoke together," from Latin com "with, together" (see com-) + bini "two by two," adverb from bi- "twice" (from PIE root *dwo- "two"). Intransitive sense "unite, coalesce, come together into one body" is from 1712. Related: Combinative; combined; combining.

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employ (v.)
early 15c., "apply or devote (something to some purpose); expend or spend," from Old French emploiier (12c.) "make use of, apply; increase; entangle; devote," from Latin implicare "enfold, involve, be connected with, unite, associate," from assimilated form of in- (from PIE root *en "in") + plicare "to fold" (from PIE root *plek- "to plait").

Imply, which is the same word, retains more of the original sense. Sense of "hire, engage" first recorded in English 1580s, from meaning "involve in a particular purpose," which arose in Late Latin. Related: Employed; employing; employable.
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mate (n.1)

mid-14c., "associate, fellow, comrade;" late 14c.,"habitual companion, friend;" from Middle Low German mate, gemate "one eating at the same table, messmate," from Proto-Germanic *ga-matjon, meaning "(one) having food (*matiz) together (*ga-)." For *matiz, see meat. It is built on the same notion as companion (which is thought to be a loan-translation from Germanic). Cognate with German Maat "mate," Dutch maat "partner, colleague, friend."

Meaning "one of a wedded pair" is attested from 1540s. Used as a form of address by sailors, laborers, etc., at least since mid-15c. Meaning "officer on a merchant vessel" is from late 15c.; his duty is to oversee the execution of the orders of the master or commander.

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partner (n.)

c. 1300, partiner, "a sharer or partaker in anything," altered from parcener (late 13c.), from Old French parçonier "partner, associate; joint owner, joint heir," from parçon "partition, division. portion, share, lot," from Latin partitionem (nominative partitio) "a sharing, partition, division, distribution" from past participle stem of partire "to part, divide" (from pars "a part, piece, a share," from PIE root *pere- (2) "to grant, allot").

The form in English has been influenced by part (n.). The word also may represent Old French part tenour "part holder." From late 14c. as "one who shares power or authority with another;" the commercial sense is by 1520s. Meaning "a husband or wife, one associated in marriage with another" is from 1749.

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mannerism (n.)

"excessive or monotonous use of distinctive methods in art or literature," 1803, from manner + -ism. Meaning "an instance of mannerism, habitual peculiarity in deportment, speech, or execution" is from 1819. Related: Mannerisms.

Perhaps few of those who write much escape from the temptation to trade on tricks of which they have learnt the effectiveness; & it is true that it is a delicate matter to discern where a peculiarity ceases to be an element in the individuality that readers associate pleasantly with the writer they like, & becomes a recurrent & looked-for & dreaded irritation. But at least it is well for every writer to realize that, for his as for other people's mannerisms, there is a point at which that transformation does take place. [Fowler]
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rival (n.)

1570s, "one who is in pursuit of the same object as another;" 1640s, "one who emulates or strives to equal or exceed another" in some way; from Latin rivalis "a rival, adversary in love; neighbor," originally, "of the same brook," from rivus "brook" (from PIE root *rei- "to run, flow").

The sense evolution seems to be based on the competitiveness of neighbors: "one who uses the same stream," or "one on the opposite side of the stream," hence in various ways "one who is in pursuit of the same object or resource as another."

A secondary sense in Latin and sometimes in English was "associate, companion in duty," from the notion of "one having a common right or privilege with another." As an adjective, "having the same pretentions or claims, holding the position of rivals," 1580s, from the noun.

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crescent (n.)

late 14c., cressaunt, "crescent-shaped ornament," from Anglo-French cressaunt, from Old French creissant, croisant "crescent of the moon" (12c., Modern French croissant), from Latin crescentum (nominative crescens), present participle of crescere "come forth, spring up, grow, thrive, swell, increase in numbers or strength," from PIE root *ker- (2) "to grow."

Applied in Latin to the waxing moon, luna crescens, but subsequently in Latin mistaken to refer to the shape, not the stage. The original Latin sense is preserved in crescendo.

Meaning "moon's shape in its first or last quarter" is from mid-15c. in English. Meaning "small roll of bread made in the form of a crescent" is from 1886. Adjectival sense of "shaped like the crescent moon" is from c. 1600 (earlier it meant "increasing, growing," 1570s).

A badge or emblem of the Turkish sultans (probably chosen for its suggestion of "increase"); figurative sense of "Muslim political power" is from 1580s, but modern writers often falsely associate it with the Saracens of the Crusades or the Moors of Spain. Horns of the waxing moon are on the viewer's left side; those of the waning moon are on his right.

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boy (n.)

mid-13c., boie "servant, commoner, knave" (generally young and male); c. 1300, "rascal, ruffian, knave; urchin," mid-14c. as "male child before puberty" (possibly an extended sense from the "urchin" one). A word of unknown origin.

Possibly from Old French embuie "one fettered," from Vulgar Latin *imboiare, from Latin boia "leg iron, yoke, leather collar," from Greek boeiai dorai "ox hides." (Words for "boy" double as "servant, attendant" across the Indo-European map -- compare Italian ragazzo, French garçon, Greek pais, Middle English knave, Old Church Slavonic otroku -- and often it is difficult to say which meaning came first.)

But it also appears to be identical with East Frisian boi "young gentleman," and perhaps with Dutch boef "knave," from Middle Dutch boeve, perhaps from Middle Low German buobe. This suggests a gradational relationship to babe. For a different conjecture: Used slightingly of young men in Middle English, also in familiar or contemptuous use of criminal toughs or men in the armed services. In some local uses "a man," without reference to age (OED lists "in Cornwall, in Ireland, in the far West of the U.S."). Meaning "male negro slave or Asian personal servant of any age" attested from c. 1600. Extended form boyo is attested from 1870. Emphatic exclamation oh, boy is attested by 1917. Boy-meets-girl "typical of a conventional romance" is from 1945; the phrase itself is from 1934 as a dramatic formula. Boy-crazy "eager to associate with males" is from 1923.

In Old English, only the proper name Boia has been recorded. ME boi meant 'churl, servant' and (rarely) 'devil.' In texts, the meaning 'male child' does not antedate 1400. ModE boy looks like a semantic blend of an onomatopoeic word for an evil spirit (*boi) and a baby word for 'brother' (*bo). [Liberman]
A noticable number of the modern words for 'boy', 'girl', and 'child' were originally colloquial nicknames, derogatory or whimsical, in part endearing, and finally commonplace. These, as is natural, are of the most diverse, and in part obscure, origin. [Buck]
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slang (n.)

1756, "special vocabulary of tramps or thieves," later "jargon of a particular profession" (1801). The sense of "very informal language characterized by vividness and novelty" is by 1818.

Anatoly Liberman writes here an extensive account of the established origin of the word from the Northern England noun slang "a narrow piece of land running up between other and larger divisions of ground" and the verb slanger "linger, go slowly," which is of Scandinavian origin (compare Norwegian slenge "hang loose, sling, sway, dangle," Danish slænge "to throw, sling"). "Their common denominator seems to be 'to move freely in any direction' " [Liberman]. Noun derivatives of these (Danish slænget, Norwegian slenget) mean "a gang, a band," and Liberman compares Old Norse slangi "tramp" and slangr "going astray" (used of sheep). He writes:

It is not uncommon to associate the place designated for a certain group and those who live there with that group’s language. John Fielding and the early writers who knew the noun slang used the phrase slang patter, as though that patter were a kind of talk belonging to some territory.

So the sense evolution would be from slang "a piece of delimited territory" to "the territory used by tramps for their wandering," to "their camping ground," and finally to "the language used there." The sense shift then passes through itinerant merchants:

Hawkers use a special vocabulary and a special intonation when advertising their wares (think of modern auctioneers), and many disparaging, derisive names characterize their speech; charlatan and quack are among them.

Liberman concludes: 

[Slang] is a dialectal word that reached London from the north and for a long time retained the traces of its low origin. The route was from "territory; turf" to "those who advertise and sell their wares on such a territory," to "the patter used in advertising the wares," and to "vulgar language" (later to “any colorful, informal way of expression”).

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[S]lang is a conscious offence against some conventional standard of propriety. A mere vulgarism is not slang, except when it is purposely adopted, and acquires an artificial currency, among some class of persons to whom it is not native. The other distinctive feature of slang is that it is neither part of the ordinary language, nor an attempt to supply its deficiencies. The slang word is a deliberate substitute for a word of the vernacular, just as the characters of a cipher are substitutes for the letters of the alphabet, or as a nickname is a substitute for a personal name. [Henry Bradley, from "Slang," in Encyclopedia Britannica, 11th ed.]

A word that ought to have survived is slangwhanger (1807, American English) "noisy or abusive talker or writer."

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