The usual word for “son” in Sindarin, derived from the root √YON of similar meaning (MR/373; SD/129; VT50/18; Ety/YŌ). Tolkien gave it as both ion and ionn.
Conceptual Development: In the Gnomish Lexicon of the 1910s the word for “son” was G. bo or bon (GL/23). This became ᴱN. gó “son” in Early Noldorin Word-lists of the 1920s (PE13/144). Tolkien introduced N. ionn “son” in The Etymologies of the 1930s derived from the root ᴹ√YO(N) of the same meaning (Ety/YŌ), and seems to have stuck with it thereafter.
Neo-Sindarin: In later writings, Tolkien sometimes glossed its Quenya equivalents yondo or yonyo as “boy” (PE17/190; VT47/10, 27). Since we don’t have any good Sindarin words for “boy”, I’d use ionn for this purpose as well.
The fifth line of Ae Adar Nín, Tolkien’s Sindarin translation of the Lord’s Prayer (VT44/21). The first word is the preposition bo “on” (possibly a mutated form for po), followed by ceven “earth”. The third word is the preposition sui “as”, followed by vi the lenited form of mi “in” and menel “heaven”. There is no Sindarin equivalent for English “it is” in this phrase.
See the entry for the first line of this prayer for a discussion of the (mis)use of menel for “Heaven” in this phrase.
Decomposition: Broken into its constituent elements, this phrase would be:
> bo Ceven sui vi Menel = “✱on Earth as in Heaven”