Adûnaic was the language of the Dúnedain of Númenor.

Adûnaic originated from the language on the peoples of the House of Hador, and is related to the languages of the Edain, as well as that of the Men of the Vales of Anduin.

It was created in the First Age and was somewhat altered by Elvish, due to the people's contact with the Moriquendi and the Eldar.

Adûnaic was the basis for the language of the later-developed Westron, but fell out of use when Westron became the more universally accepted language. The knowledge and use of it was probably maintained though by the rulers of Gondor and Arnor.
Encyclopedia entry originally written by Reasonably_Crazy