'adultness' originates from the Latin word 'adultus,' which is the past participle of 'adolescere,' meaning 'to grow up.'

'adultus' transformed into the Old French word 'adulte,' and eventually became the modern English word 'adult' with the suffix '-ness' added to form 'adultness.'

Initially, it meant 'grown up,' and over time it evolved into its current meaning of 'the state of being an adult.'