An adjective for “strong in body” in The Etymologies of the 1930s based on Old Noldorin belda “strong” under the root ᴹ√BEL “strong” (Ety/BEL). Ordinarily this Old Noldorin word would have become *bell (e.g. N. goll “wise” from ᴹ✶ñgolda), but this is one of the rare cases in Noldorin of the 1930s where final ld > lð > lt rather than the usual ll.

Neo-Sindarin: There is no sign of this sound change in later Sindarin words, so you may want to adapt this word as *bell “strong” for purposes of Neo-Sindarin, but personally I’d just keep belt and assume it was a dialectical variant or had a different primitive form.

Conceptual Development: The Gnomish Lexicon of the 1910s had G. polmog or polwed “strong (physically)”, adjective forms of G. polm or polwin “strength (physical)” (GL/64).