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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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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praeter- 

from Latin adverb and preposition praeter "beyond, past, besides, except" (from PIE root *per- (1) "forward," hence "in front of, before"). See preter-, which now is the usual form of it in English; also see æ (1).

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im- 

variant of in- before -b-, -m-, -p- in the sense of "not, opposite of" (immobile, impersonal; see in- (2)) as well as "in, into" (implant, impoverish; see in- (1)). In some English words it alternates with em- (1).

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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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aetio- 

word-forming element used in chemistry and indicating "a fundamental degradation product of a complex organic compound" [Flood], from Latinized combining form of Greek aitia "a cause, an origin" (see etiology). In older, general use it has been reduced in English to etio- (see æ (1)).

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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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petro- (1)

before vowels petr-, word-forming element used from 19c., indicating "rock, stone" (in anatomy it refers to the petrous parts of certain bones), from Greek petros "stone, boulder," petra "rock, rocky mountain range, cliff" (see petrous).

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wh- 

respelling of Old English hw- attested from 11c., but not the common form until after c. 1400. It represents PIE *kw-; in German reduced to simple w-, in Scandinavian as hv-, kv-, or v-.

It also was added unetymologically to some borrowed words (whisk, whiskey) and some native words formerly spelled with simple w- or h- (whole, whore). In the 15c. flowering of its use it also threatened to change the spelling of hot, home and many more. In northern English 16c.-18c., sometimes altered to quh- (see Q). Proper pronunciation has been much in dispute in educated speech.

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nano- 

introduced 1947 (at 14th conference of the Union Internationale de Chimie) as a prefix for units of one thousand-millionth part (now "one-billionth"), from Greek nanos "a dwarf." According to Watkins, this is originally "little old man," from nannos "uncle," masc. of nanna "aunt" (see nana), but Beekes calls it "An onomatopoeic word of unknown origin." Earlier nano- was used as a prefix to mean "dwarf, dwarfish," and still it is used in a non-scientific sense of "very small."

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