- adj. gloomy, morose, or melancholy; irritable or ill-tempered
Today’s word makes more sense if one is aware of the ancient ideas of “humours,” or the bodily substances which affect mood and disposition (a study known as “physiology.” The Latin atra bilis, or “black bile,” is what caused melancholy or sadness which present in excess. In the 1600s, the word first appeared in English, having been taken more or less directly from the Latin, adding the common suffix -ous to the end.
What is even more interesting is that the word’s Greek counterpart is μέλας (melas) and χολή (kholé), literally “black bile,” from which we derive the word “melancholy”.
The latter part of the definition (irritable) is a more recent development, the difference between plain old “bilious” (pertaining to yellow bile, which in mediæval physiology was the cause of a choleric temperament) and “atrabilious” was somewhat conflated, meaning that the latter is now used to describe either, though its real meaning is exclusively the former.