Etymology
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a (1)

indefinite article, the form of an used before consonants, mid-12c., a weakened form of Old English an "one" (see an). The disappearance of the -n- before consonants was mostly complete by mid-14c. After c. 1600 the -n- also began to vanish before words beginning with a sounded -h-; it still is retained by many writers before unaccented syllables in h- or (e)u- but is now no longer normally spoken as such. The -n- also lingered (especially in southern England dialect) before -w- and -y- through 15c.

It also is used before nouns of singular number and a few plural nouns when few or great many is interposed.

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a (2)

as in twice a day, etc., a reduced form of Old English an "on" (see on (prep.)), in this case "on each." The sense was extended from time to measure, price, place, etc. The habit of tacking a onto a gerund (as in a-hunting we will go) was archaic after 18c.

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A-1 

also A1, A-one, "first-rate," 1837 (in Dickens); a figurative use from Lloyd's of London marine insurance company's system for selective rating of merchant vessels ("Register of British and Foreign Shipping"), where it is the designation for ships in first-class condition. The letter refers to the condition of the hull of the ship itself, and the number rating to the equipment. Also used in equivalent ratings in U.S., where colloquially it is sometimes expanded to A No. 1 (which is attested by 1848 as top rating of entries in an agricultural fair).

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A.A.A. 

also AAA, abbreviation of American Automobile Association, attested 1902, American English, the year the organization was founded.

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aardvark (n.)

also aard-vark, South African groundhog, 1833 (in German from 1824), from Afrikaans Dutch aardvark, literally "earth-pig" (it burrows), from aard "earth," from Proto-Germanic *ertho- (see earth (n.)) + vark "pig," from Middle Dutch varken "small pig," which is from Proto-Germanic *farhaz (source also of Old High German farah, German Ferkel "young pig, sucking pig," a diminutive form; Old English fearh), from PIE root *porko- "young pig."

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aardwolf (n.)
Origin and meaning of aardwolf

also aard-wolf, "small, insectivorous mammal native to East and Southern Africa, related to the hyena," 1833, from Afrikaans Dutch aardwolf, literally "earth-wolf," from aard "earth" (see earth (n.)) + wolf "wolf" (see wolf (n.)).

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aba (n.)

outer garment of coarse, woolen stuff, of a type worn in Arabia and Syria, 1811, from Arabic. Also of the cloth it is made from (often goat or camel hair).

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aback (adv.)

c. 1200, "toward the rear," a contraction of Old English on bæc "backward, behind, at or on the back;" see a- (1) + back (n.). Now surviving mainly in taken aback, which originally was a nautical expression in reference to a vessel's square sails when a sudden change of wind flattens them back against the masts and stops the forward motion (1754). The figurative sense from this, "suddenly or unexpectedly checked or disappointed," is by 1792.

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abacus (n.)

late 14c., "sand table for drawing, calculating, etc.," also "art of calculating with an abacus," from Latin abacus, from Greek abax (genitive abakos) "counting table, board for drawing," a word of uncertain etymology. It is said to be from a Semitic source, such as Phoenician or Hebrew abaq "sand strewn on a surface for writing," literally "dust," from the Semitic root a-b-q "to fly off," but Beekes and others find this "semantically weak."

Originally a drawing board covered with dust or sand on which mathematical equations or calculations could be traced and erased. In reference to the other type of abacus, a counting frame with beads or balls strung on wires or rods, it is attested from 17c. or later in English. Both types were known in antiquity across Eurasia. Related: Abacist (late 14c.)

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Abaddon 

late 14c., used in Revelation ix.11 of "the angel of the bottomless pit," and by Milton of the pit itself, from Hebrew Abhaddon, literally "destruction," from abhadh "he perished." The Greek form was Apollyon.

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