"one of a race of powerful supernatural beings in Germanic folklore," Old English elf (Mercian, Kentish), ælf (Northumbrian), ylfe (plural, West Saxon), from Proto-Germanic *albiz (cf. Old Saxon alf, Old Norse alfr, German alp "evil spirit, goblin, incubus"), origin unknown, possibly from PIE *albho- "white." Used figuratively for "mischievous person" from 1550s.
In addition to elf/ælf (masc.), Old English had parallel form *elfen (fem.), the plural of which was *elfenna, -elfen, from Proto-Germanic *albinjo-. Both words survived into Middle English and were active there, the former as elf (with the vowel of the plural), plural elves, the latter as elven, West Midlands dialect alven (plural elvene).
The Germanic elf originally was dwarfish and malicious (cf. Old English ælfadl "nightmare," ælfsogoða "hiccup," thought to be caused by elves); in the Middle Ages they were confused to some degree with faeries; the more noble version begins with Spenser. Nonetheless a popular component in Anglo-Saxon names, many of which survive as modern given names and surnames, cf. Ælfræd "Elf-counsel" (Alfred), Ælfwine "Elf-friend" (Alvin), Ælfric "Elf-ruler" (Eldridge), also women's names such as Ælfflæd "Elf-beauty." Elf Lock hair tangled, especially by Queen Mab, "which it was not fortunate to disentangle" [according to Robert Nares' glossary of Shakespeare] is from 1592.