fervor

Fervor comes from Middle English fervour, borrowed from Old French, which took it from Latin fervor, meaning a boiling or raging heat, or by extension, vehemence and passion. That Latin noun grew from fervere, "to boil, be hot."

The word's core sense has stayed remarkably steady: what began as literal boiling heat in Latin became the figurative heat of passion and intensity that fervor carries in English today.

It shares its root with fervent, both built on the Latin verb fervere, "to boil, be hot."