Besides

wiktionary

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From Middle English biside, equivalent to beside +‎ -s. See -s(Etymology 3)

etymonline

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besides (prep.)

attested from c. 1200, common after c. 1400, from beside (q.v.) + adverbial genitive -s. Once sharing all the senses of beside, now properly limited to the adverbial sense "in addition to, otherwise."