spiritus

Spiritus, spiritus, m., fourth declension: genitive spiritus, dative spiritui, ablative spiritu, nominative and accusative plural spiritus. The stem is spiro, "I breathe", and the literal sense is breath. Spirit, courage and finally the Holy Spirit all grow out of air moving through a body.

The fourth declension trap bites here more than anywhere. Spiritus on the page can be nominative singular, genitive singular or nominative or accusative plural, and only the sentence around it decides. Latin also splits what English fuses: anima is the life-breath and the soul, spiritus the breath and the spirit, animus the mind and will. In this corpus spiritus sanctus swamps everything else.