Old English milde, of persons, powers, or dispositions, "possessing softness or gentleness, good-tempered, merciful," from Proto-Germanic *milthjaz- (source also of Old Norse mildr (which also contributed to the English word), Old Saxon mildi, Old Frisian milde, Middle Dutch milde, Dutch mild, Old High German milti, German milde "mild," Gothic mildiþa "kindness"), from PIE *meldh-, from root *mel- (1) "soft," which is the source also of Latin mollis "soft."

Of weather, "not rough or stormy," late 14c. Of medicine, etc., "gentle or moderate in force, operation, or effect," c. 1400; of disease from 1744. Of rule, punishment, etc., "moderate in quality or degree, of mitigated force, not hard to endure," by 1570s. It was also used in Old English as an adverb, meaning "mercifully, graciously."

Mild goes further than gentle in expressing softness of nature; it is chiefly a word of nature or character, while gentle is chiefly a word of action. [Century Dictionary]

"state or quality of being mild" in any sense, Old English mildnes "mildness, mercy," from mild (adj.) + -ness.

"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.

fem. proper name, Old English Mildðryð, from milde "mild" (see mild) + ðryð "power, strength" (see Audrey). A popular name in the Middle Ages through fame of St. Mildred (obit c. 700), abbess, daughter of a Mercian king and a Kentish princess. Familiar forms include Milly, Midge. Among the 10 most popular names for girls born in the U.S. between 1903 and 1926, it hasn't been in the top 1,000 since 1983.

expression of mild disappointment, sympathy, etc.; recorded in this form by 1888.