late 14c. (ardently is attested from mid-14c.), from O.Fr. ardant (13c.), from L. ardentem (nom. ardens), prp. of ardere "to burn," from PIE base *as- "to burn, glow" (cf. Skt. asah "ashes, dust;" Armenian azazem "I dry up;" Gk. azein "to dry up, parch;" Goth. azgo, O.E. æsce "ashes;" L. ardus "parched, dry"). The fig. sense (of passions, desire, etc.) was earliest in Eng.; literal sense of "burning, parching" (mid-15c.) remains rare. Ardent spirits (1471) "strong alcoholic liquor" so called because they are inflammable, but the term now, if used at all, probably is felt in the figurative sense.
1540s, "warmth of feeling, desire," from ardent. A figurative sense, the literal meaning "intensity of heat" wasn't attested until 1630s.