'applier' (and thus 'appliers') originates from Latin, specifically the word 'applicare', where 'ad-' (later assimilated to 'ap-') meant 'to/toward' and 'plicare' meant 'to fold' or 'to join/apply'.

'applicare' passed into Old French as 'appliquer'/'aplicier' and Middle English adopted forms based on these; English developed the verb 'apply' from these sources and formed the agent noun 'applier' by adding the suffix '-er', eventually giving the modern plural 'appliers'.

Initially, the Latin root meant 'to attach or join' and later 'to bring into contact' or 'to apply'; over time this evolved into modern senses of 'to put on, to use, or to make a formal request' and 'applier' came to mean 'one who applies' (person or device).