Bottommost means the one that is lowest of all — the furthest down, or the deepest. The bottommost drawer, the bottommost layer of rock, the bottommost step. It is a real word, but a slightly stiff or bookish one. In ordinary talk, people usually say the bottom one, the lowest, or the very bottom. You will meet it in careful writing more than in conversation. It can also mean the furthest in a “down” direction you are already picturing, not only “toward the floor”.

最底下的、最底部的