Wiktionary
sit upon
n. 1 (context British colloquial English) buttocks, bottom 2 small, portable cushion for sitting on
Usage examples of "sit upon".
From being a fat person who was comfortably placed in the shade of a tree, this poor person would become a thin person, with not much of a bottom to sit upon, and probably unable to sleep as a result.
Its cushions were decorated with shiny silver buttons that absolutely, positively could not have been comfortable to sit upon.
The floor was covered with a deep blue carpet, and large red cushions were scattered here and there to sit upon.