on Just Publishing Advice:
The origin of the idiom to vent your spleen dates back to Hippocrates in 400BC.
The belief endured until the nineteenth century that four bodily fluids controlled physical and mental qualities.
Blood, phlegm, yellow bile, and black bile were called humours. The word humour, or humor in US spelling, derives from Latin, meaning liquid or fluid.
It was yellow bile, which was thought to be responsible for anger and restlessness that the idiom vent one’s spleen derives.