*as-

early 14c., ardaunt, specifically of alcoholic distillates, brandy, etc., "flammable," from Old French ardant "burning, hot; zealous" (13c.), from Latin ardentem (nominative ardens) "glowing, fiery, hot, ablaze," also used figuratively of passions, present participle of ardere "to burn" (from PIE root *as- "to burn, glow").

The figurative sense ("burning with passions, desire, etc.") is from late 14c.; the general etymological sense of "burning, parching" (c. 1400) remains rare. Ardent spirits (late 15c.) retains the oldest English meaning, but the term now, if used at all, probably is felt in a figurative, causative sense. Related: Ardently.

"heat of passion or desire," mid-15c., ardour, from Old French ardure "heat, glow; inflammation; passion" (12c., Modern French ardeur), from Latin ardorem (nominative ardor) "a flame, fire, burning, heat;" also of feelings, etc., "eagerness, zeal," from ardere "to burn" (from PIE root *as- "to burn, glow"). In Middle English used of base passions; since Milton's time of noble ones.