'atrabilious' originates from Latin, specifically the word 'atrābilis', where 'ater' meant 'black' and 'bilis' meant 'bile'.

'atrābilis' passed into Medieval/Medieval Latin as 'atrabilis' and into English (via Middle English usage of humoral vocabulary) to become the modern English word 'atrabilious'.

Initially, it meant 'affected by black bile' (a humoral medical condition), but over time it evolved into its current meaning of 'melancholic' or 'ill-tempered'.