shit, excrement
屎 refers directly to excrement or feces, with no abstract extensions beyond the literal bodily waste meaning.
屎 combines 尸 (body) and 米 (rice), likely representing bodily waste after food has been processed. The exact historical development is uncertain, but the component combination suggests a semantic origin related to digestion.
Under the body enclosure 尸, you see rice 米 that has been processed and come out as waste. Picture the rice turning into excrement after digestion.
For シ, imagine a sheep (シ sounds like 'she') leaving droppings that look like grains of rice: sheep -> シ, and the rice inside the body enclosure becomes sheep droppings.