Truly

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Old English trēowlīce ‘faithfully’ (see true, -ly2).

wiktionary

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From Middle English truely, treuly, treuli, trewely, treoweliche, treowliche, from Old English trēowlīċe(“faithfully; truly”), equivalent to true +‎ -ly. Cognate with Dutch trouwelijk, Middle Low German truwlike, German treulich, Swedish trolig, Icelandic trygglega.

etymonline

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truly (adv.)

Old English treowlice, from treowe (see true (adj.)). Similar formation in Dutch treuwelijk, German getreulich, Swedish troligen.