'annihilatory' originates from Latin, specifically the word 'annihilare', where 'ad-' meant 'to' (often assimilated to 'an-') and 'nihil' meant 'nothing'.

'annihilare' (Latin) produced 'annihilatus' in Late Latin, passed into Middle English via the verb 'annihilen'/'annihilate' and later produced the adjective form 'annihilatory' in modern English.

Initially, it meant 'to reduce to nothing' in Latin; over time the core sense of 'causing complete destruction' has remained and is carried by the modern adjective 'annihilatory'.