The native tongue of the Númenóreans, and the ancestor of Westron. Adûnaic was a descedant of the language of the House of Hador which was enriched during the First Age by Elvish influences (it is also relative to the language of Men on the Vales of Anduin). Adûnaic was not an official language of Númenor, until it replaced the Elvish languages at the days of its Pride (II 2899 - II 3319). The Kings after Ar-Adûnakhôr took the Sceptre under Adûnaic names instead of Quenya, the use of which was banned. After its Downfall, the Exiles did not use it, and was considered a language of the low class people. However an early form of it, spoken in Middle-earth during the Númenórean travels, evolved to Westron, the language considered the Common Speech throughout the Westlands.... Probably called in its own term Adûnaiyê.
For further information, structure and grammar see also http://www.uib.no/People/hnohf/adunaic.htm