'ascertain' originates from Old French, specifically the word 'acertener' (or Anglo-French forms), where the prefix/root 'ad-' (in later Latinized forms) meant 'to/ toward' and 'certus' meant 'sure' or 'fixed'.

'ascertain' changed from Vulgar Latin 'accertāre' (to make certain) into Old French 'acertener' and Middle English 'ascerteinen' before becoming the modern English 'ascertain'.

Initially it meant 'to make certain' (i.e., cause to be certain); over time it evolved to include the sense 'to find out or discover (something) for certain', which is the common modern usage.