'atramentous' originates from Latin, specifically the word 'atramentum', where 'atramentum' meant 'black ink' and ultimately from 'ater' meaning 'black'.

'atramentous' changed from Late Latin/Medieval Latin forms such as 'atramentōsus' (meaning 'full of ink') and passed into Middle English before becoming the modern English word 'atramentous'.

Initially, it meant 'of or containing atrament (black ink)', but over time it has come to mean more generally 'very black; like ink', with occasional figurative uses meaning 'dark' or 'gloomy'.