"of a steady temper," 1650s, from Latin aequanimis "mild, kind" (see equanimity) + -ous.
Entries linking to equanimous
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.
-ous
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."