mild (adj.) Look up mild at Dictionary.com
Old English milde "gentle, merciful," from Proto-Germanic *milthjaz- (source also of Old Norse mildr, Old Saxon mildi, Old Frisian milde, Middle Dutch milde, Dutch mild, Old High German milti, German milde "mild," Gothic mildiþa "kindness").

This is from PIE *meldh-, from the root *mel- (1) "soft," with derivatives referring to soft or softened materials (source also of Hittite mallanzi "they grind;" Armenian malem "I crush, bruise;" Sanskrit mrdh "to neglect," also "to be moist;" Greek malakos "soft," malthon "weakling," mylos "millstone," myle "mill;" Latin molere "to grind," mola "millstone, mill," milium "millet;" Old Irish meldach "tender;" Old English melu "meal, flour;" Albanian miel "meal, flour;" Old Church Slavonic meljo, Lithuanian malu "to grind;" Old Church Slavonic mlatu, Russian molotu "hammer").

Originally of persons and powers; of the weather from c. 1400, of disease from 1744. Also in Old English as an adverb, "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]