The usual Sindarin patronymic suffix, meaning “son of” (PE17/170; Ety/YŌ), a suffixal form of ion(n) “son”. It was occasionally used for “descendants”, especially as a class plural, as in Hurinionath “Descendants of Húrin” (PM/202).
Conceptual Development: In Gnomish, the prefix G. go- or gon- (suffixal -iod, -ion, -ios) was initially used with the meaning “son of” as in Gon Indor “✱Son of Indor” (LT2/217), but in the Gnomish Lexicon of the 1910s the prefix was revised to G. bo- or bon-, as in Tuor bo-Beleg, along with suffixal -von or -mon (GL/23, 40-41). Tolkien reintroduced suffixal N. -ion “son” in The Etymologies of the 1930s under the root ᴹ√YO(N) “son” (Ety/YŌ), and seems to have stuck with it thereafter.
This suffix is frequently used to create the adjective form of a noun, especially in the form -ëa for nouns ending in -ë. This function dates back to CE. ✶-ā.