ᴺS. [N.] ^pathu n. “level space, sward” (Category: Plain, Field)
A noun appearing as N. pathw “level place, sward” in The Etymologies of the 1930s derived from primitive ᴹ✶pathmā, where the m became w after the aspirate th as in ON. pathwa (Ety/PATH). This final -w would be pronounced “-u”, and having -w in the written form seems to be an orthographic convention of Noldorin of the 1930s and 40s. In (Neo) Sindarin it would probably be written as ᴺS. pathu as suggested in Hiswelókë’s Sindarin Dictionary (HSD/pathu); compare for example S. hadhu “seat”.
References ✧ Ety/PATH; EtyAC/PATH
|ON. pathwa > pathw||[paθwa] > [paθw] > [paθu]||✧ Ety/PATH|