ᴺS. [N.] ^daer [nd-] n. “bridegroom” (Category: Husband)
A noun appearing as N. doer “bridegroom” in The Etymologies of the 1930s derived from the root ᴹ√NDER of similar meaning (Ety/NDER). It is unusual in that its primitive form is ᴹ✶ndǣr, a rare example of the a-fortification of primitive e to ǣ (PE18/46). In Old Noldorin this became ON. ndair, and in Noldorin of the 1930s the diphthong [ai] became [oe] or [ae]. Indeed, in another entry in The Etymologies, Tolkien gave a variant form daer for “bridegroom”, though somewhat mysteriously he marked it as Old Noldorin (Ety/DER).
Neo-Sindarin: The a-fortification of primitive e remained a feature in Tolkien’s later writings, though in the 1950s Tolkien marked the result as ę̄ rather than ǣ (PE18/95). Thus primitive √NDER > *ndę̄r > OS. ndair > S. daer remains a plausible scenario in Sindarin, but ai > oe no longer occurred as it did in Noldorin. Therefore, I’d use the form ᴺS. daer for “bridegroom” in Neo-Sindarin, as suggested in Hiswelókë’s Sindarin Dictionary (HSD).
References ✧ Ety/DER, NDER
|ᴹ√NDERE > daer||[ndǣr] > [ndair] > [dair] > [daer]||✧ Ety/DER|
|ON. ndair > doer||[ndair] > [dair] > [doer]||✧ Ety/NDER|