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a-

1
  1. a reduced form of the Old English preposition on, meaning “on,” “in,” “into,” “to,” “toward,” preserved before a noun in a prepositional phrase, forming a predicate adjective or an adverbial element (afoot; abed; ashore; aside; away), or before an adjective (afar; aloud; alow), as a moribund prefix with a verb (acknowledge), and in archaic and dialectal use before a present participle in -ing (set the bells aringing); and added to a verb stem with the force of a present participle (ablaze; agape; aglow; astride; and originally, awry).
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Origin of a-

1
Middle English, late Old English; cf. a2, nowadays

a-

2
  1. a reduced form of the Old English preposition of: akin; afresh; anew.
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Origin of a-

2
Middle English; see a3

a-

3
  1. an old point-action prefix, not referring to an act as a whole, but only to the beginning or end: She arose (rose up). They abided by their beliefs (remained faithful to the end).
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Origin of a-

3
Middle English; Old English a- (unstressed), ǣ-, ā-, ō- (stressed; see abb, woof1, oakum), rarely or- (see ordeal) ≪ Germanic *uz- < unstressed Indo-European *uss- < *ud-s, akin to out; in some cases confused with a-4, as in abridge

a-

4
  1. variant of ab- before p and v: aperient; avert.
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Origin of a-

4
Middle English < Latin ā-, a- (variant of ab- ab-); in some words < French a- < Latin ab-, as in abridge

a-

5
  1. variant of ad-, used: (1) before sc, sp, st (ascend) and (2) in words of French derivation (often with the sense of increase, addition): amass.
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Origin of a-

5
Middle English, in some words < Middle French a- < Latin ad- prefix or ad preposition (see ad-), as in abut; in others < Latin a- (variant of ad- ad-), as in ascend

a-

6
  1. variant of an-1 before a consonant, meaning “not,” “without”: amoral; atonal; achromatic.
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A-

  1. atomic (used in combination): A-bomb; A-plant.
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for a-

a-

1

before a vowel an-

prefix
  1. not; without; opposite toatonal; asocial
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Word Origin for a-

from Greek a-, an- not, without

a-

2
prefix
  1. on; in; towardsafoot; abed; aground; aback
  2. literary, or archaic (used before a present participle) in the act or process ofcome a-running; go a-hunting
  3. in the condition or state ofafloat; alive; asleep
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for a-

1

in native (derived from Old English) words, it most commonly represents Old English an "on" (see a (2)), as in alive, asleep, abroad, afoot, etc., forming adjectives and adverbs from nouns; but it also can be Middle English of, as in anew, abreast (1590s); or a reduced form of Old English past participle prefix ge-, as in aware; or the Old English intensive a-, as in arise, awake, ashame, marking a verb as momentary, a single event. In words from Romanic languages, often it represents Latin ad- "to, at."

[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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2

prefix meaning "not," from Latin a-, short for ab "away from" (e.g. avert), or its cognate, Greek a-, short for apo "away from, from," both cognate with Sanskrit apa "away from," Gothic af, Old English of (see apo-).

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3

prefix meaning "not," from Greek a-, an- "not," from PIE root *ne "not" (see un-).

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Online Etymology Dictionary, © 2010 Douglas Harper

a- in Medicine

a-

pref.
  1. Without; not:acellular.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

a- in Science

a-

  1. A prefix meaning “without” or “not” when forming an adjective (such as amorphous, without form, or atypical, not typical), and “absence of” when forming a noun (such as arrhythmia, absence of rhythm). Before a vowel or h it becomes an- (as in anhydrous, anoxia).
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.