rotten meat, bones of dead animals
胔 centers on the remains of dead animals: rotten meat and bones. It describes what is left after death and decay.
胔 combines 肉 (flesh/meat) with 此, which likely contributes the sound. The exact historical development is uncertain.
This (此) meat (肉) is rotten—see the bones and decay.
For シ, imagine a scene of death: the 'she' (シ) of the dead animal is just rotten meat and bones.