PIE base ‘under, up from under, over’.
Other derivatives include up, above, often, sub, open, substance, souvenir, supple, succeed, and many others. Also Sanskrit Upanishad ‘a sitting down beside’ (a guru), and Upanishads, scriptures on the unity of Brahman and Atman.
[Pokorny *upo-, up, eup, (e)up-s-, pp. 1106–1107.]