'doomsdayism' is formed in modern English by combining the noun 'doomsday' with the suffix '-ism' (which denotes a doctrine or belief).

'doomsday' comes from Old English 'dōmes dæg' (literally 'day of judgment'), which became Middle English 'domesday'/'domes dai' and later modern English 'doomsday'. The suffix '-ism' derives from Greek '-ismos' via Latin and French and was attached to English nouns to indicate systems of belief or practice.

Initially, 'doomsday' meant 'the day of judgment'; over time it broadened to mean 'the end of the world' in general, and 'doomsdayism' has come to mean the belief or ideology that such an end is imminent or inevitable.