equanimity (n.)

c. 1600, "fairness, impartiality," from French équanimité, from Latin aequanimitatem (nominative aequanimitas) "evenness of mind, calmness; good-will, kindness," from aequanimis "mild, kind," literally "even-minded," from aequus "even, level" (see equal (adj.)) + animus "mind, spirit" (see animus). Meaning "evenness of temper" in English is from 1610s.