Biliousness stems from ancient humoral theory, which held that the body’s health depended on the balance of bodily fluids, including bile. Excess bile was believed to produce irritability, digestive complaints, and a sallow complexion. In literature the word often describes both bodily discomfort and a gloomy or irritable mood. Poets sometimes employ biliousness to evoke the unpleasant physical effects of illness or to characterise a temperament coloured by bitterness, reflecting the old belief that emotions and bodily humours were deeply connected.

Biliousness comes from historical medical theories about bodily humours, where excess bile was thought to produce irritability and digestive discomfort. In poetry the term often represents ill temper, bitterness, or sour disposition. Writers sometimes use it humorously to mock gloomy or complaining characters. It also appears metaphorically to describe landscapes, politics, or moods coloured by resentment or unpleasantness. Understanding humour theory helps students appreciate how older literature linked emotional temperament with physical bodily fluids.