URL: 

bottoms up mud sample

1. n. [Drilling Fluids]
A sample of mud from the deepest or current drilling depth of a well. The term refers particularly to a mud sample that has experienced stagnant conditions at the bottom of the hole, including the temperature, pressure and other conditions at that depth. A bottoms-up sample is commonly collected after a trip out of the hole or if an influx of formation fluid is suspected
Alternate Form: bottoms-up mud sample