1. vb. 
The act of passing the mud around a piece of equipment, such as passing mud returns around the shale shaker screens or going around a hydrocyclone device. From a mud-engineering viewpoint, this can be a bad practice because it can allow drill solids to degrade and accumulate as fines to the degree that they might cause mud problems.
2. n. 
A system of pipes and valves permitting the diversion of flow or pressure around a line valve.