'ardentness' originates from English, formed from the adjective 'ardent' and the suffix '-ness', where 'ardent' ultimately comes from Latin 'ardēns' (present participle of 'ardēre') meaning 'burning', and the suffix '-ness' derives from Old English '-nes(s)' meaning 'state or quality'.

'ardent' passed into English via Old French 'ardant' from Latin 'ardēns/ardēre'; the noun 'ardentness' was created in English by adding the native suffix '-ness' to the adjective 'ardent'.

Initially, Latin 'ardēre' meant 'to burn'; over time the sense shifted metaphorically to 'burning with emotion', and the modern 'ardentness' now means 'intense passion or enthusiasm'.