1. n. 
The mud flowing into the well. The mud-in sample is taken from the suction pit (the last pit in the flow series) just before the mud goes into the pump and down the wellbore. The in sample is also called the suction-pit sample, or "mud in" on a drilling fluid report. This mud has been treated and properly weighted and is in good condition to encounter downhole pressures, temperatures and contamination. Comparisons are made between properties of this mud-in sample and the "out" or mud-out sample taken at surface prior to solids removal.
See: mud-in sample