Fully

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Old English fullīce (see full1, -ly2).

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From Middle English fully, fulliche, volliche, from Old English fullīċe(“fully”), equivalent to full +‎ -ly. Compare German völlig(“fully”), Swedish fullt(“fully”).

etymonline

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fully (adv.)

Old English fullice "entirely; perfectly; completely;" see full (adj.) + -ly (2). Of similar formation are Dutch vollijk, German ööllig, Danish fuldelig.