'aptness' originates from Late Middle English, formed from the adjective 'apt' plus the suffix '-ness'; 'apt' ultimately comes from Latin 'aptus', where 'aptus' meant 'fitted' or 'suitable'.

'aptness' changed from Middle English 'aptnesse' (or 'aptnesse') and eventually became the modern English word 'aptness' through regularization of spelling and suffix usage.

Initially, it meant 'being fitted or suitable' (from Latin 'aptus'); over time it broadened to include both general 'suitability' and the sense of 'natural quickness or aptitude' in modern usage.