Slowly

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Old English slāw ‘slow-witted, sluggish’, of Germanic origin.

wiktionary

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From Middle English slowly, slowli, slouli, slowliche, from Old English slāwlīċe(“slowly; sluggishly”), equivalent to slow +‎ -ly. Compare Old Norse slæliga, sljóliga.

etymonline

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slowly (adv.)

Old English slawlice "slowly, sluggishly;" see slow (adj.) + -ly (2).