'atrabilarious' originates from Medieval Latin, specifically the word 'atrabiliosus', where 'ater/atra' meant 'black' and 'bilis' meant 'bile'.

'atrabilarious' changed from the Medieval Latin word 'atrabiliosus' and the related adjective 'atrabilious' in Early Modern English; 'atrabilarious' appears as a rare or dialectal variant alongside 'atrabilious' and both relate back to the same Latin source.

Initially it meant 'relating to black bile or a melancholic temperament' in pre-modern humoral theory; over time it evolved into a general descriptor for 'melancholic, sullen, or ill-tempered' and is now archaic/rare.