Senya

After the defeat of Sauron at the end of the Second Age, the victorious Isildur, now High King of the Realms in Exile, set out for Annúminas with his elder sons and the captured Ring. As they journeyed north, the King's company was waylaid by a horde of Orcs, and Isildur's eldest son Elendur urged his father to use Sauron's Ring, addressing him as atarinya, or 'my father'. In response Isildur explained that he could not control the Ring's power, using the word senya to address his son.

The word senya is not explained, and remains something of a mystery. From the context of its sole known use, 'my son' would seem a likely translation, but that would normally be onya, which is indeed the word seen in similar situations elsewhere. The Elvish root sen- is normally connected with resting or being at peace, so hardly seems to apply in this frantic situation. One possible interpretation would be 'my child' (via Quenya seldo, 'child') though it is impossible to be sure whether this was Tolkien's actual intention.