Because

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Middle English: from the phrase by cause, influenced by Old French par cause de ‘by reason of’.

wiktionary

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From Middle English bi cause, from bi(“by”) + cause, modelled on Old French par cause.

etymonline

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because (conj.)

late 14c., from phrase bi cause, introducing a subordinate clause or phrase, "by cause, for the reason that," from by (prep.) + cause (n.). Modeled on French par cause. Originally often followed by that or why. As an adverb, "by reason, on account" (with of), from late 14c. Clipped form cause (sometimes 'cause) is attested in writing by mid-15c.