Although

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Middle English: from all (as an adverb) + though.

wiktionary

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From Middle English althagh, from Old English compound of eall(“all (emphatic)”) + þēah(“though”)

etymonline

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although (conj.)

early 14c., althagh, contraction of all though, preserving the once-common emphatic use of all. "All though was originally more emphatic than though, but by 1400 it was practically only a variant of it, and all having thus lost its independent force, the phrase was written as one word" [OED]. The choice between though and although is often determined by rhythm.