'inexactness' originates from Latin and Old English: the element 'exact' comes from Latin 'exactus' (past participle of 'exigere'), where 'ex-' meant 'out' and 'agere' meant 'to drive'; the negative prefix 'in-' is from Latin 'in-' meaning 'not'; the suffix '-ness' comes from Old English '-ness' used to form nouns.

'inexactness' developed by combining the negative prefix 'in-' with the adjective 'exact' (borrowed into Middle English from Old French 'exact', from Latin 'exactus'), and then adding the noun-forming suffix '-ness' to create the modern English noun 'inexactness'.

Originally related to Latin 'exactus' meaning 'driven out' or 'precise' (via notions of exact measurement or demand), the negated and nominalized form came to mean 'the state of not being exact'—i.e., lack of precision.