torus

Of uncertain origin.

De Vaan proposes a tentative derivation from a Proto-Indo-European *torh₂-os, from *terh₂- (“to cross, go through”). This is formally and semantically solid, though there is little linguistic evidence to support it.

An older theory by Leumann derived the word from sternō (“to spread”), itself from Proto-Indo-European *sterh₃- (“to spread, extend”), but the connection between the two Latin words is semantically dubious (one would expect torus to be derived from a verb meaning "to turn" or "to knot" rather than "to spread").