'inestimability' originates from Latin via Old French and Middle English, specifically the Latin components 'in-' + 'aestimare', where 'in-' meant 'not' and 'aestimare' meant 'to value or assess'.

'inestimability' developed from Late Latin/Old French forms such as 'inestimabilis' (meaning 'not able to be valued') and passed into Middle English as 'inestimable' with the noun-forming suffix '-ity' to become 'inestimability' in modern English.

Initially it meant 'not able to be valued or assessed', and over time it has retained that sense while also being used to emphasize 'pricelessness' or 'immeasurable worth' in modern usage.