'ascertain' originates from Anglo-Norman/Old French, specifically the word 'acerteiner' (or Middle English 'ascerten'), where the prefix 'ad-/as-' (assimilated form) meant 'to/toward' and 'certain' meant 'sure, settled'.

'ascertain' changed from Old French 'acerteiner' into Middle English 'ascerten' and eventually became the modern English verb 'ascertain'; from this came the adjective 'ascertainable' and the adverb 'ascertainably'.

Initially it meant 'to make certain or ensure'; over time it evolved to mean 'to find out or determine (a fact)', and derived forms came to mean 'able to be determined' (ascertainable) and 'in a way that can be determined' (ascertainably).