'astonishingness' ultimately derives from the verb 'astonish', which comes from Old French 'estoner' (or 'estoner' in Anglo-Norman), itself from Vulgar Latin *'extonāre' composed of Latin prefix 'ex-' meaning 'out' and 'tonare' meaning 'to thunder'.

'astonish' entered English via Anglo-Norman/Middle English (forms such as 'astounen' or 'astonien') from Old French 'estoner'; the noun forms (e.g. 'astonishment') and later abstract formations (like 'astonishingness') developed in Modern English.

Originally related to being 'stunned' or 'thunder-struck' (literally 'struck by thunder' in the Latin compound), the sense shifted to general strong surprise or amazement, and modern derivatives denote that quality or degree of surprise.