PIE base ‘to sit’.
Other derivatives include to set, settle, sit, seat, chair, nest, niche, reside, polyhedron, and Sanskrit Upanishad ‘a sitting down beside’ (a guru), and Upanishads, scriptures on the unity of Brahman and Atman.
[Pokorny *sed- ‘to sit, set, settle’, pp. 884–887.]