furthermore (adv.)

Old English furðor, forðor "to a more advanced position, forward, onward, beyond, more distant; farther away; later, afterward; to a greater degree or extent, in addition; moreover," etymologically representing either "forth-er" or "fore-ther." The former would be from furðum (see forth) + comparative suffix *-eron-, *-uron- (compare inner, outer).

Alternative etymology (Watkins) traces it to Proto-Germanic *furthera-, from PIE *pr-tero- (source also of Greek proteros "former"), representing the root *per- (1) "forward" + comparative suffix also found in after, other. Senses of "in addition, to a greater extent" are later metaphoric developments.

It replaced or absorbed farrer, ferrer as comparative of far (itself a comparative but no longer felt as one). Farrer itself displaced Old English fierr in this job; farrer survived until 17c., then was reduced to dialect by rival farther. "The primary sense of further, farther is 'more forward, more onward'; but this sense is practically coincident with that of the comparative degree of far, where the latter word refers to real or attributed motion in some particular direction." [OED]

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).

comparative word-forming element added to already comparative adjectives and adverbs, Middle English (innermore, outermore, furthermore, overmore, etc.), from more (adv.). The formation also was in Old Norse and the English use might be from Scandinavian.