"each, considered indefinitely as a unitary part of an aggregate; all, of a collective or aggregate number, taken one by one;" early 13c., contraction of Old English æfre ælc "each of a group," literally "ever each" (Chaucer's everich), from each with ever added before it for emphasis. The word still is felt to want emphasis; as in Modern English every last ..., every single ..., etc.

Also a pronoun to Chaucer, Shakespeare, Spenser, "each of any number of persons or things; every one." Compare everybody, everything, etc. The word everywhen is attested from 1843 but never caught on; neither did everyhow (1837).

Every now and then "repeatedly, at short intervals" is from 1660s. Every once in a while, U.S. colloquial, "now and then, from time to time," is attested from 1814 (Bartlett calls it "A singular though very common expression"). Slang phrase every Tom, Dick, and Harry "every man, everyone" dates from at least 1723, from the common English given names.

That is to ſay, they affirm, that once upon a Time (tho' they never yet could tell when) all Mankind were upon a Level, and that there was no ſuch Thing as Government in the World; and that Tom, Dick, and Harry, ay, every individual Man, Woman, and Child, had a Right to the whole World. [Charles Leslie, "A Short and Eaſie Method with the Deists," London, 1723]

"every person, everybody," c. 1200, from every + one.

name of the leading character in a popular 15c. morality play; from every + man (n.).

"every person, every individual of a body or mass of persons," late 14c., from every + body (n.) in obsolete sense of "person."

"all things, taken separately; any total or aggregate considered with reference to its constituent parts; each separate item or particular," late 14c., from every + thing. Colloquially, "something of extreme importance," by 1889.

1630s, "worn on ordinary days," as opposed to Sundays or high days, from noun meaning "a week day" (late 14c.), from every (adj.) + day (n.). Extended sense of "to be met with every day, common" is from 1763.

Old English ælc (n., pron., adj.) "any, all, every, each (one)," short for a-gelic "ever alike," from a "ever" (see aye (adv.)) + gelic "alike" (see like (adj.)). From a common West Germanic expression *aina-galīk (source also of Dutch elk, Old Frisian ellik, Old High German iogilih, German jeglich "each, every"). Originally used as we now use every (which is a compound of each) or all; modern use is by influence of Latin quisque. Modern spelling appeared late 1500s. Also see ilk, such, which.

also *ayu-, Proto-Indo-European root meaning "vital force, life; long life, eternity."

It forms all or part of: age; aught (n.1) "something; anything;" aye (adv.) "always, ever;" Ayurvedic; coetaneous; coeval; each; eon; eternal; eternity; ever; every; ewigkeit; hygiene; longevity; medieval; nay; never; no; primeval; sempiternal; tarnation; utopia. 

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit ayu- "life;" Avestan aiiu "age, life(time);" Greek aiōn "age, vital force; a period of existence, a lifetime, a generation; a long space of time," in plural, "eternity;" Latin aevum "space of time, eternity;" Gothic aiws "age, eternity," Old Norse ævi "lifetime," German ewig "everlasting," Old English a "ever, always."  

"in every place, in all places," c. 1200, eauerihwer, contracted from Old English æfre gehwær; see ever (adv.) + where. Not from every; the -i- in the word apparently was a prefix; compare handiwork. Old English had also æghwær " 'aywhere,' everywhere."

late 15c., "every hour;" as an adjective, 1510s, "happening or done every hour," from hour + -ly.