Possibly

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late Middle English: from Old French, or from Latin possibilis, from posse ‘be able’.

wiktionary

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possible +‎  -ly

etymonline

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possibly (adv.)

c. 1400, possibli, "by any existing power or means, in a possible manner," from possible (adj.) + -ly (2). By c. 1600 as "perhaps. perchance."