From Tolkien Gateway

Atarinya is an Elvish term meaning 'my father'. It is recorded twice in Tolkien's work, used once by Elendur to his father Isildur,[1] and also long before by Aldarion to his father Meneldur in Númenor.[2] In both cases, the opposite form would be onya, 'my son'. The same word-ending could be used to create affectionate forms from personal names: for example, Meneldur also called Aldarion Anardilya ('my Anardil' - with Anardil being Tar-Aldarion's original name).[2]