aen is a Sindarin word which probably means "should", "rather", "they" or generally a passive voice marker; its exact meaning, usage and etymology is unclear.
The word aen is seen in the phrase of the King's Letter
aen follows the verb estathar which is future tense and means "they will name". It is not known if aen can follow verbs of other tenses. Likewise, it is not known if aen is the pronoun ("they should name him Fullwiise"); a marker of the passive voice ("He should be called Fullwise"); or an optative marker which expresses a wish ("Whom instead should call Fullwise").
Another riddle is the word sennui which is also of unknown meaning, with possibilities being "should" or "instead".
- Hiswelókë's Sindarin dictionary, see entries for aen and sennui