'ardency' ultimately originates from Latin, specifically from the verb 'ardēre' meaning 'to burn', via the adjective 'ardent' (from Latin 'ardēns'/'ardent-') and noun-forming suffixes that produced English 'ardency'.

'ardēre' (Latin) produced the present-participial/adjectival form 'ardēns' → Old French/Anglo-Norman 'ardent' → Middle English 'ardent', and the noun form developed (influenced by Late Latin 'ardentia') into modern English 'ardency'.

Initially it referred to literal 'burning' or heat from the Latin root; over time the sense became metaphorical, coming to mean 'intense passion or fervor', which is its current meaning.