Equanimity is a lovely word.
Equanimity, meaning fairness, impartiality, came to English around 1600 from French
quanimité, Latin
aequanimatatem (evenness of mind, calmness; good will, kindness), and Latin
aequanimus (mild, kind; even-minded) from
aequus (even, level) +
animus (mind, spirit). Latin
aequus is also the source of the word equal.
Latin
animus embodies several meanings: rational soul, mind, life, mental powers, intelligence, consciousness, sensibility; courage, desire.
Animus is related to Latin
anima (living being, soul, mind, disposition, passion, courage, anger, spirit, feeling) from the Proto-Indo-European root
ane (to breathe).
Equanimity meaning evenness of temper is from the 1610s. The English word animus, meaning temper in a hostile sense, is from 1820.
Reference: Online Etymological Dictionary,
https://www.etymonline.com/