Etymology
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kn- 
Middle English spelling of a common Germanic consonant-cluster (in Old English it was graphed as cn-; see K). The sound it represented persists in most of the sister languages, but in English it was reduced to "n-" in standard pronunciation by 1750, after about a century of weakening and fading. It was fully voiced in Old and Middle English.
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sch 

consonant cluster that can represent five distinct sounds in English; it first was used by Middle English writers to render Old English sc-, a sound now generally pronounced (and spelled) "-sh-." Sometimes it was miswritten for ch. It also was taken in from German (schnapps) and Yiddish (schlemiel). In words derived from classical languages (school (n.1)), it represents Latin sch-, Greek skh-, but in some of these words the spelling is a restoration and the pronunciation does not follow it (as in schism; Middle English sisme, cisme).

The Yiddish words with it, often derisive or dismissive, tended to come into 20c. American English. In addition to those with entries here, Saul Bellow used schmegeggy, "Portnoy's Complaint" has schmatte "a ragged garment;" schmeck "a sniff" figures in heroin jargon, and schmutz "dirt, filth" has been used. Directly to English from German also are some specialized words: Schmelz "enamel," schmerz "grief, pain, sorrow," 

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com- 
Origin and meaning of com-

word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (compare Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.

Before vowels and aspirates, it is reduced to co-; before -g-, it is assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, and -v-, it is assimilated to con-, which was so frequent that it often was used as the normal form.

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atmo- 

word-forming element meaning "vapor," from Greek atmos "vapor, steam," which is of uncertain origin. Watkins has it from PIE *awet-mo-, from root *wet- (1) "to blow" (also "to inspire, spiritually arouse;" see wood (adj.)). Beekes says it is not considered to be related to the source of atman.

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caco- 
before vowels cac-, word-forming element meaning "bad, ill, poor" (as in cacography, the opposite of calligraphy and orthography), from Latinized form of Greek kakos "bad, evil," considered by etymologists probably to be connected with PIE root *kakka- "to defecate." The ancient Greek word was common in compounds; when added to words already bad, it made them worse; when added to words signifying something good, it often implies too little of it.
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di- (1)

word-forming element of Greek origin meaning "two, double, twice, twofold," from Greek di-, shortened form of dis "twice," which is related to duo "two" and cognate with bi-, from PIE root *dwo- "two." In chemistry it indicates a compound containing two units of the element or radical to which it is prefixed.

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eco- 
word-forming element referring to the environment and man's relation to it, abstracted from ecology, ecological; attested from 1969.
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anti- 

word-forming element of Greek origin meaning "against, opposed to, opposite of, instead," shortened to ant- before vowels and -h-, from Old French anti- and directly from Latin anti-, from Greek anti (prep.) "over, against, opposite; instead, in the place of; as good as; at the price of; for the sake of; compared with; in opposition to; in return; counter-," from PIE *anti "against," also "in front of, before" (from root *ant- "front, forehead," with derivatives meaning "in front of, before"), which became anti- in Italian (hence antipasto) and French.

It is cognate with Sanskrit anti "over, against," and Old English and- (the first element in answer). A common compounding element in Greek, in some combinations it became anth- for euphonic reasons. It appears in some words in Middle English but was not commonly used in English word formations until modern times. In a few English words (anticipate, antique) it represents Latin ante.

In noun compounds where it has the sense of "opposed to, opposite" (Antichrist, anti-communist) the accent remains on the anti-; in adjectives where it retains its old prepositional sense "against, opposed to," the accent remains on the other element (anti-Christian, anti-slavery).

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a- (1)

prefix or inseparable particle, a conglomerate of various Germanic and Latin elements.

In words derived from Old English, it commonly represents Old English an "on, in, into" (see on (prep.)), as in alive, above, asleep, aback, abroad, afoot, ashore, ahead, abed, aside, obsolete arank "in rank and file," etc., forming adjectives and adverbs from nouns, with the notion "in, at; engaged in." In this use it is identical to a (2).

It also can represent Middle English of (prep.) "off, from," as in anew, afresh, akin, abreast. Or it can be a reduced form of the Old English past participle prefix ge-, as in aware.

Or it can be the Old English intensive a-, originally ar- (cognate with German er- and probably implying originally "motion away from"), as in abide, arise, awake, ashamed, marking a verb as momentary, a single event. Such words sometimes were refashioned in early modern English as though the prefix were Latin (accursed, allay, affright are examples).

In words from Romanic languages, often it represents reduced forms of Latin ad "to, toward; for" (see ad-), or ab "from, away, off" (see ab-); both of which by about 7c. had been reduced to a in the ancestor of Old French. In a few cases it represents Latin ex.

[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose. [OED]
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pur- 

Middle English and Anglo-French perfective prefix, corresponding to Old French por-, pur- (Modern French pour), from Vulgar Latin *por-, a variant of Latin pro "before, for" (see pro-). This is the earliest form of the prefix in English, and it is retained in some words, but in others it has been corrected to Latinate pro-.

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