fervor (n.)

mid-14c., "warmth or glow of feeling," from Old French fervor "heat; enthusiasm, ardor, passion" (12c., Modern French ferveur), from Latin fervor "a boiling, violent heat; passion, ardor, fury," from fervere "to boil; be hot" (from PIE root *bhreu- "to boil, bubble, effervesce, burn").