to bury, to stop up, dirty, filthy
薶 centers on covering or concealing something, often in a negative sense: burying, stopping up, or making dirty. The meanings share the idea of something being hidden or obscured, whether physically buried or metaphorically soiled.
薶 is a phono-semantic compound: 艹 (grass) suggests covering or burying under vegetation, while 貍 likely contributes the sound. The exact historical development is uncertain.
Grass 艹 grows over a hidden animal 貍, burying it and making the ground dirty and stopped up.
For バイ, imagine buying a plot of land to bury something under the grass: buy -> バイ, and the grass covers the buried object.