Mildly

wiktionary

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mild +‎  -ly

etymonline

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mildly (adv.)

"in a mild manner or degree," Middle English mildeli, from Old English mildelice "graciously, affably, kindly;" see mild + -ly (2). Compare Dutch mildlijk, German mildlich, Danish mildelig. Phrase to put it mildly, implying a harsher reality than stated, is attested by 1849.