'armageddonist' originates from Modern English, specifically formed from the word 'Armageddon' plus the suffix '-ist' (denoting an adherent or believer). 'Armageddon' itself comes via Greek 'Armagedōn' from the Hebrew phrase 'Har Megiddo' meaning 'mountain (or hill) of Megiddo.'

'Armageddon' changed from Hebrew 'Har Megiddo' → Greek 'Armagedōn' → entered Late Latin/Medieval Christian usage and then Modern English as 'Armageddon'. The modern English noun 'armageddonist' was created by adding the agentive suffix '-ist' to 'Armageddon'.

Initially, 'Armageddon' referred to a geographic place (the hill/valley of Megiddo) and the associated biblical battle; over time it came to signify the prophesied catastrophic end of the world, and 'armageddonist' evolved to mean someone who believes in or predicts such an apocalypse.