Old English ebba "falling of the tide, low tide," perhaps from Proto-Germanic *af- (source also of Old Frisian ebba, Old Saxon ebbiunga, Middle Dutch ebbe, Dutch eb, German Ebbe), from PIE root *apo- "off, away." Figurative sense of "decline, decay, gradual diminution" is from late 14c. Ebb-tide is from 1776.

Old English ebbian "flow back, subside," from the root of ebb (n.). Figurative use in late Old English. Related: Ebbed; ebbing.

Middle English awei, from late Old English aweg, earlier on weg "on from this (that) place;" see a- (1) + way (n.).

The meaning "from one's own or accustomed place" is from c. 1300; that of "from one state or condition to another" is from mid-14c.; that of "from one's possession (give away, throw away) is from c. 1400. Colloquial use for "without delay" (fire away, also right away) is from the earlier sense of "onward in time" (16c.). The meaning "at such a distance" (a mile away) is by 1712. Intensive use (as in away back) is American English, attested by 1818. Of sporting events played at the other team's field or court, by 1893.

also lay-away, 1961 in reference to a system of payments for reserved merchandise, from the verbal phrase (attested from c. 1400 as "to put away," especially "place in store for future use"); see lay (v.) + away (adv.). Earlier in the same sense, as an adjective, was Australian lay-by (1930).

also getaway, 1852, "an escape," originally in fox hunting, from verbal phrase get away "escape" (early 14c.); see get (v.) + away (adv.). Of prisoners or criminals from 1893.

as a game, 1925, from verbal phrase (attested from late 14c.); see keep (v.) + away (adv.).

also giveaway, "act of giving away," 1872, from verbal phrase give away, c. 1400 (of brides from 1719); see give (v.) + away (adv.). The phrase in the meaning "to betray, expose, reveal" is from 1878, originally U.S. slang. Hence also Related: give-away (n.) "inadvertent betrayal or revelation" (1882).

also faraway, "distant, remote," 1816, from far + away.

of coats, "cut back from the waist," 1841, from the verbal phrase; see cut (v.) + away. As a noun, "coat cut back from the waist," by 1849. In reference to models, drawings, etc., of which a part is cut away to reveal the interior, by 1946. The verbal phrase is from c. 1300 as "cut (something) off or away." 

also *ap-, Proto-Indo-European root meaning "off, away."

It forms all or part of: ab-; abaft; ablaut; aft; after; apanthropy; aperitif; aperture; apo-; apocalypse; apocryphal; Apollyon; apology; apoplexy; apostle; apostrophe; apothecary; apotheosis; awk; awkward; ebb; eftsoons; of; off; offal; overt.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit apa "away from," Avestan apa "away from," Greek apo "from, away from; after; in descent from," Latin ab "away from, from," Gothic af, Old English of "away from."