The phrase "vent one's spleen" originates from ancient Greek and Roman medical theories known as humorism. According to this theory, the spleen was believed to be the organ responsible for producing and storing black bile, one of the four humors or bodily fluids. This idiom may be used in various occasions, such as during heated debates, arguments, or discussions where individuals passionately express their discontent or dissatisfaction. It can also be used in personal settings, such as venting frustrations to a trusted friend or family member, or even in written form, like venting through social media posts or blog entries.