Means ‘Lord of the West’ in Adûnaic or Númenórean. His name in Quenya is Herunúmen also meaning ‘Lord of the West’.

He was the twentieth King of Númenor and the first to take his name in the Adûnaic tongue. In the Scroll of Kings though, his name was inscribed as Herunúmen because the kings dared not utterly forsake the ancient custom for fear of evil befalling their people.

The Faithful felt that this name was over-proud because it was a title of the Valar to whom only they now remained loyal. Ar-Adûnakhôr had forsaken the Elven-tongues and forbid the use of them where he could hear it. During his reign he began to persecute The Faithful and would punish any who used the Elven-tongues openly. So it was that the Eldar came no more to Númenor.
Encyclopedia entry originally written by Nienna-of-the-Valar