'ammonia-less' is a modern compound formed from 'ammonia' + the adjectival suffix '-less' (from Old English 'lēas'), literally meaning 'without ammonia'. 'Ammonia' itself comes into English via New/Medieval Latin 'ammonia' (from 'sal ammoniac'), ultimately named after the Egyptian god Amun (Ammon) and the ammonium-bearing deposits associated with his oracle.

'ammonia' passed from Medieval/Neo-Latin 'ammonia' (and 'sal ammoniac') into Modern English as 'ammonia'; the suffix '-less' comes from Old English 'lēas' → Middle English '-les' → modern '-less'; the compound 'ammonia-less' is a recent English formation combining these elements to describe the absence of ammonia.

Originally 'ammonia' referred to salts associated with the temple of Ammon (a geographic/ritual association); over time it became the chemical name for the compound NH3. The suffix '-less' has consistently meant 'without', so the compound now straightforwardly means 'without ammonia.'