every (adj.)

"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]

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Definitions of every from WordNet

every (adj.)
(used of count nouns) each and all of the members of a group considered singly and without exception;
every chance of winning
every party is welcome
every person is mortal
had every hope of success
every (adj.)
each and all of a series of entities or intervals as specified;
every third seat
every two hours