hypothesis

Recorded in English since 1596, from Middle French hypothese, from Late Latin hypothesis, from Ancient Greek hupóthesis, meaning 'base, basis of an argument, supposition' — literally 'a placing under.'

The Greek word came from hupotíthēmi, 'to set before, suggest,' itself built from hupó ('below') and títhēmi ('to put, place') — so the sense of a foundation set beneath an argument has carried through largely unchanged into its use for a proposed explanation awaiting proof.

It's worth comparing with hypothec, another word that shares this Greek root about placing something underneath as a foundation.