early 13c., contraction of Old English æfre ælc "each of a group," literally "ever each" (Chaucer's everich), from each with ever added for emphasis, as the word is still felt to need emphasis (e.g. Modern English every last ..., every single ..., etc.).
Cf. everybody, everything, etc. The word everywhen is attested from 1843 but never caught on; neither did everyhow (1837). Slang phrase every Tom, Dick, and Harry dates from at least 1734, from common English given names.