Old English eall "every, entire, the whole quantity of" (adj.), "fully, wholly, entirely" (adv.), from Proto-Germanic *alnaz (source also of Old Frisian, Old High German al; German all, alle; Old Norse allr; Gothic alls), with no certain connection outside Germanic. As a noun, in Old English, "all that is, everything."
Combinations with all meaning "wholly, without limit" were common in Old English (such as eall-halig "all-holy," eall-mihtig "all-mighty") and the habit has continued. Middle English had al-wher "wherever; whenever" (early 14c.); al-soon "as soon as possible," al-what (c. 1300) "all sorts of things, whatever."
The use of a, a' as an abbreviation of all (as in Burns' "A Man's a Man for A' that") is a modern Scottishism but has history in English to 13c.
Of the common modern phrases with it, at all "in any way" is from mid-14c., and all "and everything (else)" is from 1530s, all but "everything short of" is from 1590s. First record of all out "to one's full powers" is by 1880. All clear as a signal of "no danger" is recorded from 1902. All right, indicative of assent or approval, is attested by 1837; the meaning "satisfactory, acceptable" is by 1939, from the notion of "turning out well."
All's fair in love and war is by 1826 with that wording; but variants (all advantages are lawful in love and war, in love and war all stratagems are fair, etc.) are to be found as far back as 17c. in English. The phrase may have originated with Don Quixote:
...así como en la guerra es cosa lícita y acostumbrada usar de ardides y estratagemas para vencer al enemigo, así en las contiendas y competencias amorosas se tienen por buenos los embustes y marañas que se hacen para conseguir el fin que se desea,
como no sean en menoscabo y deshonra de la cosa amada
. [Miguel de Cervantes,
Don Quixote, 1605.]
...For as in War, so in Love, Stratagems are always allow'd. [Translation by J. Philips, 1687]
