'assignably' ultimately originates from Latin, specifically the verb 'assignare', where 'ad-' (often reduced to 'as-') meant 'to' and 'signare' meant 'to mark or sign'.

'assignare' passed into Old French as 'assigner' and Middle English as 'assignen'/'assigne', becoming the modern English verb 'assign'; the adjective 'assignable' was formed with the suffix '-able', and the adverb 'assignably' followed by adding '-ly'.

Initially, the root meant 'to mark out or allot'; over time it evolved into the modern sense 'to allocate, appoint, or transfer', and 'assignably' now means 'in a way that can be assigned or transferred'.