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]
expression of mild disappointment, sympathy, etc.; recorded in this form by 1888.
In Middle English, used of the weather. Current sense of "easily flexible" is from c. 1300. Related: Litheness. Old and Middle English had the related verb lin "to cease doing (something)," also used of the wind dying down.
mid-15c., of persons, "mild in temper or disposition" (attested from early 13c. as a surname), from Old French clement, from Latin clementem (nominative clemens) "mild, placid, gentle" (see clemency). Of weather, 1620s. Taken as a name by several early popes and popular in England as a masculine given name from mid-12c., also in fem. form Clemence.
1650s, "relaxing, soothing" (a sense now archaic), from French lenient, from Latin lenientem (nominative leniens), present participle of lenire "to soften, alleviate, allay; calm, soothe, pacify," from lenis "mild, gentle, calm," which probably is from a suffixed form of PIE root *lē- "to let go, slacken."
The usual modern sense of "mild, merciful" (of persons or actions) is first recorded 1787. In earlier use was lenitive, attested from early 15c. of medicines, 1610s of persons. Related: Leniently.