equanimity n.

late 14c., "identical in amount, extent, or portion;" early 15c., "even or smooth of surface," from Latin aequalis "uniform, identical, equal," from aequus "level, even, flat; as tall as, on a level with; friendly, kind, just, fair, equitable, impartial; proportionate; calm, tranquil," which is of unknown origin. Parallel formation egal (from Old French egal) was in use late 14c.-17c. Equal rights is from 1752; by 1854 in American English in reference to men and women. Equal opportunity (adj.) in terms of hiring, etc. is recorded by 1925.

1820, "temper" (usually in a hostile sense), from Latin animus "rational soul, mind, life, mental powers, consciousness, sensibility; courage, desire," related to anima "living being, soul, mind, disposition, passion, courage, anger, spirit, feeling," from PIE root *ane- "to breathe."

It has no plural. As a term in Jungian psychology for the masculine component of a feminine personality, it dates from 1923 (compare anima). For sense development in Latin, compare Old Norse andi "breath, breathing; current of air; aspiration in speech;" also "soul, spirit, spiritual being."