1A hole containing mud or in which mud collects, especially one which forms a defect or obstacle in a road; specifically a hot mud spring.
2A hole at the base of a boiler, condenser, etc., through which sediment can be removed.
3figurative A despised place; a place of filth and squalor.
4A situation or factor that impedes progress or entraps a person.
Typically used in the context of an extended metaphor.
Early 18th century. From mud + hole.