It has no plural. As a term in Jungian psychology for the masculine component of a feminine personality, it dates from 1923. For sense development in Latin, compare Old Norse andi "breath, breathing; current of air; aspiration in speech; soul, spirit, spiritual being."
1580s, "nobly brave or valiant," from magnanimity + -ous, or else from Latin magnanimus "highminded," literally "great-souled," from magnus "great" (from PIE root *meg- "great") + animus "mind, soul, spirit" (see animus). From 1590s as "elevated in soul or sentiment, superior to petty resentments." Related: Magnanimously.
early 15c., pusillanimus, "timid, lacking strength and firmness of mind," from Late Latin pusillanimis "having little courage" (used in Church Latin to translate Greek oligopsykhos "small-souled"), from Latin pusillis "very weak, little" (diminutive of pullus "young animal," from PIE root *pau- (1) "few, little") + animus "spirit, courage" (see animus). Related: Pusillanimously; pusillanimousness.