c. 1300 as one word, "at all times; all the time; forever, eternally;" see ever + more. Replacing evermo (13c.), from Old English æfre ma.
Entries linking to evermore
ever adv.
Old English æfre "ever, at any time, always;" of uncertain origin, no cognates in any other Germanic language; perhaps a contraction of a in feore, literally "ever in life" (the expression a to fore is common in Old English writings). First element is almost certainly related to Old English a "always, ever," from Proto-Germanic *aiwi-, extended form of PIE root *aiw- "vital force, life; long life, eternity." Liberman suggests second element is comparative adjectival suffix -re.
Sometimes contracted to e'er in dialect and poetry. Ever began to be used in late Old English as a way to generalize or intensify when, what, where, etc. The sense evolution was from "at any time at all, in any way" to "at any particular time; at some time or another; under any circumstances." Ever so "to whatever extent" is recorded by 1680s. Expression did you ever? (implying "see/do/hear of such a thing") attested by 1840.
more adj.
Old English mara "greater, relatively greater, more, stronger, mightier," used as a comparative of micel "great" (see mickle), from Proto-Germanic *maiz (source also of Old Saxon mera, Old Norse meiri, Old Frisian mara, Middle Dutch mere, Old High German meriro, German mehr, Gothic maiza), from PIE *meis- (source also of Avestan mazja "greater," Old Irish mor "great," Welsh mawr "great," Greek -moros "great," Oscan mais "more"), perhaps from a root *me- "big."
Sometimes used as an adverb in Old English ("in addition"), but Old English generally used related ma "more" as adverb and noun. This became Middle English mo, but more in this sense began to predominate in later Middle English.
"Take some more tea," the March Hare said to Alice, very earnestly.
"I've had nothing yet," Alice replied in an offended tone, "so I can't take more."
"You mean you can't take less," said the Hatter: "it's very easy to take more than nothing."
As a noun, "a greater quantity, amount, or number," in Old English. More and more "larger and larger amounts" is from 12c. More or less "in a greater or lesser degree" is from early 13c.; appended to a statement to indicate nearness but not precision, from 1580s. The more the merrier "the larger the company the greater the enjoyment" is from late 14c. (þe mo þe myryer).