A Sindarin word appearing as {hrū >} rhû “matter” in Quenya Notes (QN) from 1957, cognate to Q. hrón “flesh/substance of Arda, matter” and derived from srōn < √SRON, a root variation of √RON “solid, tangible, firm”. In later writings Tolkien revised its Quenya cognate to {orma} > erma (MR/218, 231 note #26, 338, 359 note #14).
Neo-Sindarin: I would retain S. rhû for “matter”, and would assume it was of independent derivation from its Quenya equivalent.
n. matter. rhû << rhū. >> rhond, rhonn