'anti-' originates from Greek, specifically the word 'anti', where 'anti' meant 'against'; 'gas' originates from Dutch, specifically the word 'gas', coined in the 17th century.

'anti-gas' formed in English by combining the prefix 'anti-' (from Greek via Latin/Old French into English) with 'gas' (from Dutch 'gas'); the compound usage became particularly noted in the 20th century with the development of chemical warfare countermeasures.

Initially it simply meant 'against gas' (literally opposing or countering gas); over time it came to refer specifically to agents, devices, or measures that neutralize or protect against toxic/chemical gases.