'applicably' ultimately originates from Latin, specifically the verb 'applicare', where 'ad-' (expressed in the prefix) meant 'to, toward' and 'plicare' meant 'to fold or bend', giving the sense 'to attach or apply'.

'applicare' passed into Old French as 'appliquer' and into Middle English forms related to 'applicable'; the adjective 'applicable' (from Medieval Latin 'applicabilis') later took the adverbial suffix '-ly' to form 'applicably'.

Initially it was associated with the physical sense 'to attach or join', but over time it evolved into the abstract sense 'to be relevant or appropriate', which is the basis of the modern adverbial use 'applicably'.