Strongly

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Old English, of Germanic origin; related to Dutch and German streng, also to string.

wiktionary

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From Middle English strongliche, strangliche, from Old English stranglīċe(“strongly”), equivalent to strong +‎ -ly.

etymonline

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strongly (adv.)

Old English stranglice "firmly, stoutly, boldly, bravely;" see strong (adj.) + -ly (2).