early 14c., of alcoholic distillates, brandy (ardent spirits), etc., from Old French ardant (13c.) "burning, hot; zealous," 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" from PIE root *as- "to burn, glow" (cognates: Old English æsce "ashes;" see ash (n.1)).
Ardent spirits (late 15c.) so called because they are inflammable, but the term now, if used at all, probably is felt in the figurative sense. The figurative sense (of "burning with" passions, desire, etc.) is from late 14c.; literal sense of "burning, parching" (c. 1400) remains rare. Related: Ardently.