Origins
The word 'equanimity' entered English in the early seventeenth century from Latin 'aequanimitās,' meaning 'evenness of mind,' 'calmness,' or 'composure.' The Latin compound joins 'aequus' (equal, even, level) with 'animus' (mind, spirit, soul), creating a word that means literally 'equal-mindedness' or 'level-spiritedness' — a state in which the mind remains balanced, neither elated by good fortune nor depressed by bad.
The Latin element 'aequus' (equal, level, fair) generated a large English family through French and direct Latin borrowing. 'Equal' (the same in quantity or status), 'equity' (fairness, impartiality), 'equator' (the line that divides the Earth into two equal halves), 'equinox' (equal night — the day when day and night are of equal length), 'equivalent' (of equal value), 'equidistant' (equally distant), 'equilibrium' (equal balance), 'equitable' (fair), and 'adequate' (made equal to, sufficient) all derive from 'aequus.'
The element 'animus' (mind, spirit, courage) has a more surprising etymology. It comes from PIE *h₂enh₁- (to breathe), reflecting the ancient and widespread identification of the soul or spirit with breath. When you breathe, you are animated — filled with 'anima' (Latin for 'breath,' 'soul,' 'life force'). Latin 'anima' (breath, soul) and 'animus' (mind, spirit) are related nouns from the same root. English derivatives include 'animate' (to give life/breath to), 'animal' (a breathing/living being), 'animation,' 'inanimate' (lifeless, not breathing), 'unanimous' (of one mind/spirit — 'unus' + 'animus'), 'magnanimous' (great-spirited), 'pusillanimous' (small-spirited, cowardly), and 'animosity' (hostile spirit).
Proto-Indo-European Roots
Greek 'ánemos' (ἄνεμος, wind) is a cognate — wind as the breath of the world. English 'anemone' (the wind-flower) comes from this Greek word. Sanskrit 'ātman' (soul, self, breath) — a central concept in Hindu philosophy — descends from the same PIE root, as does Old English 'ǣþm' (breath).
Equanimity as a philosophical ideal has deep roots in both Western and Eastern traditions. The Stoic philosophers (Zeno, Epictetus, Marcus Aurelius, Seneca) made equanimity the cornerstone of their ethics: the sage was one who maintained an even mind regardless of external circumstances, neither disturbed by loss nor inflated by gain. Marcus Aurelius's 'Meditations' is essentially a manual for achieving equanimity.
In Buddhism, the concept of 'upekkhā' (Pali) or 'upekṣā' (Sanskrit) — usually translated as 'equanimity' — is one of the four 'Brahmavihāras' or 'sublime states,' alongside loving-kindness, compassion, and sympathetic joy. Buddhist equanimity involves maintaining balanced awareness without attachment or aversion — a state remarkably similar to the Stoic ideal, arrived at independently.
Latin Roots
The word itself, with its Latinate formality and its four syllables, carries a gravity appropriate to the concept it names. Equanimity is not indifference (which implies not caring) or apathy (which implies not feeling). It is the maintenance of an even keel in the face of what would normally disturb — a quality that requires not less emotion but more mastery.