c.1300, from O.Fr. as "one at dice," from L. as (gen. assis) "a unit," from the name of a small Roman coin, perhaps originally Etruscan and related to Gk. eis "one." It meant the side of the die with only one mark before it meant the playing card. Since this was the lowest roll at dice, ace was used metaphorically in M.E. for "bad luck;" but as the ace is often the highest playing card, the extended senses based on "excellence, good quality" arose 18c. as card-playing became popular. Meaning "outstanding pilot" dates from 1917 (technically, in WWI aviators' jargon, one who has brought down 10 enemy planes, though originally in ref. to 5 shot down), from Fr. l'ace (1915), which, according to Bruce Robertson (ed.) "Air Aces of the 1914-1918 War" was used in prewar Fr. sporting publications for "top of the deck" boxers, cyclists, etc. Sports meaning of "point scored" (1819) led to that of "unreturnable serve" (1889). The verb meaning "to score" (in sports) is first attested 1923, and led to the extended student slang sense of "get high marks" (1959). Ace in the hole "concealed advantage" is attested from 1915.