Is it ensure, insure, or assure?
severe, unrelenting; cruel
Latin in- + clementem 'mild'
1660s, from French inclément and directly from Latin inclementem (nominative inclemens) "harsh, unmerciful," from in- "not, opposite of, without" (see in- (1)) + clementem "mild, placid." "Limitation to weather is curious" [Weekley].