A general agental suffix that developed from the ancient suffix ✶-ro, mainly when final r became syllabic -or after other consonants. This can be seen most clearly in [N.] tavor “woodpecker, [lit.] knocker” derived from older tafr [tavr] < ᴹ✶tamrō (Ety/TAM).
Conceptual Development: This agental suffix also appeared as G. -or in the Gnomish Lexicon of the 1910s in words such as G. nandor “farmer” and G. ecthor “swordsman”. It appeared in a quite a few words in The Etymologies of the 1930s. There is some evidence of it in Tolkien’s later writings, such as S. magor “swordsman” attested in S. Menelvagor “Swordsman of the Sky” (LotR/81), and in Notes on Names (NN) from 1957 Tolkien said -or was a variant of the masculine agental suffix -on used after n (PE17/141). This variant had a plural form -yr.
Neo-Sindarin: For purposes of Neo-Sindarin, most people assume -or is a gender-neutral agental suffix.
A (masculine) agental suffix appearing as either -on or -ron. The -on variant is derived from ancient ✶-on(do), as seen in [N.] callon “hero” < ᴹ✶kalrondō (Ety/KAL). The -ron variant arose from the addition of ✶-on(do) to the ancient agental suffix ✶-ro after vowel losses made that suffix indistinct. This can be seen most clearly in [N.] thavron “carpenter” < [ON.] sthabro(ndo) < ᴹ✶stabrō (Ety/STAB). Other times the reduced -r became syllabic -or, as in [N.] tavor “woodpecker, [lit.] knocker” < tafr [tavr] < ᴹ✶tamrō (Ety/TAM).
Sometimes the suffix -(r)on was specifically masculine, as in Ellon vs. Elleth “Elf (m./f.)” and [N.] odhron vs. odhril “parent (m./f.)” (WJ/363; Ety/ONO). Other times it seems to be more neutral in meaning, as in pethron “speaker” or mellon “friend” (PE18/100). As a suffix in names, -on is only masculine.
Conceptual Development: The suffix G. -(r)on dates all the way back to the Gnomish Lexicon of the 1910s, but in Early Noldorin Word lists of the 1920s, it usually appeared as ᴱN. -ion.