Partly

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late Middle English (in partial (sense 2 of the adjective)): from Old French parcial (partial (sense 2 of the adjective)), French partiel (partial (sense 1 of the adjective)), from late Latin partialis, from pars, part- ‘part’.

wiktionary

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part +‎  -ly

etymonline

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partly (adv.)

"in part, in some measure or degree, not wholly," 1520s, from part (n.) + -ly (2).