Q. Greetings from New Zealand. May I please ask you what is the plural of “thesis” and whether this word is of Latin or Greek origin? Many thanks.
A. For this sort of thing, we at the University of Chicago Press must rely on the lexicographers. The plural, according to both Webster’s tenth and American Heritage, is “theses.” As for origin, according to the Oxford English Dictionary, “thesis” was
[o]riginally and properly, according to ancient writers, [t]he setting down of the foot or lowering of the hand in beating time, and hence (as marked by this) the stress or ictus; the stressed syllable of a foot in a verse; a stressed note in music.
Among the ancient writers offered as proof are the authors of Greek fragments studied by Rudolf Westphal in his Die Fragmente und die Lehrsätze der griechischen Rhythmiker from 1861. Later Latin writers inverted this meaning, but the one that has generally survived today is the setting down, not of the foot, but of a proposition. So Greek it is.