'Thesis' is Greek for 'something placed down' — from PIE *dheh- (to put). A proposition for argument.
A statement or theory that is put forward as a premise to be maintained or proved; a long essay or dissertation involving personal research, written as part of a university degree.
From Latin 'thesis,' from Greek 'thesis' (a placing, a proposition, a setting down), from 'tithenai' (to place, to set, to put), from PIE *dheh₁- meaning 'to put, to set, to place.' This root is one of the most productive in Proto-Indo-European, generating Latin 'facere' (to do, to make), Sanskrit 'dadhati' (he places), Old English 'don' (to do), and Greek 'tithenai.' The Greek noun 'thesis' meant any act of placing — in music, the downbeat; in grammar, the position of a word; in philosophy, a proposition set down for argument. English inherited the academic sense — a proposition to be defended — through Medieval Latin university culture, entering scholarly use in the 14th century and general use thereafter. Key roots: *dʰeh₁- (Proto-Indo-European: "to put, to place, to set").
English 'thesis,' 'do,' and 'deed' are all from PIE *dʰeh₁- (to put, to place). Greek kept the 'putting/placing' sense (thesis = something placed). Germanic shifted to the more general 'doing' sense (do = to perform an action, deed = something done). A thesis is literally something you 'do' — you place an idea before an audience.