Some scholars believe this word to be cognate to Quenya nai "may it be". Other think that it is the pronoun "they". The context in which this word occurs (The so-called "King's Letter") is not clear enough to intepret it with certainty
Sindarin
aen
adverb. *should, could
aen
(unknown meaning)
aen
aen
The word is not easy to analyze. If its meaning is "should", there is a possibility that it is a cognate or related to the Quenya expression nai, "may it be that".
aenor
noun. god
aeni
noun. (female) angel
@@@ phonetic adaptation from Quenya
aenu
noun. (male) angel
@@@ phonetic adaptation from Quenya
Fui
noun. (unknown meaning)
ain
adverb. *should, could
glithui
adjective. (unknown meaning)
Eru
god
(the One) #Eru, isolated from Eruchín** **"children of the One" (= Elves and Men; sg. *Eruchen).
eru
noun. God
eru
god
isolated from Eruchín "children of the One" (= Elves and Men; sg. ✱Eruchen).
The exact meaning of aen in King’s Letter has long been a source of speculation. It is generally thought to be some sort of subjunctive marker (“should”) or other marker of uncertainty, the earliest idea along these lines being Carl Hostetter’s suggestion that it might be related to Q. nai “may it be” (VT31/16). The 2023 publication of the earliest draft of the letter in The Art of the Manuscript provides very strong evidence for this theory. In this earliest draft, the word was first written as ge (AotM/62), likely a mutated form of ce which could be the cognate of Q. cé “maybe” or “if” (VT49/19). This was revised to ain, likely based on primitive ✶ai “supposing” from around this period (PE22/139). This was revised to aen in all later versions of the letter, reflecting the Sindarin sound change whereby ai became ae.
Assuming this analysis is correct, then aen probably means something like “should” or “could” and the untranslated parenthetical comment i sennui Panthael estathar aen means something like “who [rather?] should be called Fullwise”, a play on Sam’s actual name Panthael = “Halfwise”.