thesis

First recorded in 1350–1400; Middle English, from Latin, from Greek thésis “a setting down, something set down,” equivalent to the- (stem of tithénai “to put, set down”) + -sis noun suffix; see -sis

Thesis comes via Latin thesis from Greek thésis. There are only two meanings of thesis in Latin: in rhetoric, a general or abstract question (as opposed to a particular case), a proposition (the pure Latin term is propositum “something put forward”); and in poetry, the unstressed part of a metrical foot. However, Greek thésis has the opposite sense, the stressed part of a metrical foot. The Greek commentators explained thésis as the downward step of the foot (as of a marching soldier) or the lowering of the hand (as of a conductor indicating the accented beat). In Late Latin thesis meant “a lowering of the voice on an unstressed syllable” (its opposite is arsis, “a raising of the voice on a stressed syllable”). The sense “unstressed syllable or note” appears in English at the end of the 14th century and has been the accepted meaning since the 18th century in English poetry and music.
Thesis with the meaning “a proposition put forward to be discussed, proved, or defended” appeared in 1579; the more specific meaning “a dissertation required for an academic degree” dates from the second half of the 17th century. These meanings developed from the rhetorical sense in Latin and Greek.

C16: via Late Latin from Greek: a placing, from tithenai to place