Old English freondlic "well-disposed, kindly;" see friend (n.) + -ly (1). Related: Friendlily; friendliness. As an adverb Old English had freondliche, but by 14c. as the inflections wore off in English it had become indistinguishable from the adjective. Probably owing to that it is rare in modern use; friendfully (mid-15c.) and the correct but ungainly friendlily (1670s) never caught on.
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Harper, D. (n.d.). Etymology of ecofriendly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/ecofriendly