1550s, "mildness or gentleness shown in exercise of authority," from L. clementia
"calmness, gentleness," from clementem
) "calm, mild," related to -clinare
"to lean" (see lean
(v.)). Meaning "mildness of weather or climate" is 1660s; clement
(adj.) is older in both senses, late 15c. and 1620s respectively, but now is used only in negation and only of the weather.