Means possibly ‘Noble Man’ for ‘ar’ = noble and ‘atan’ or ‘adan’ = man.

The second-born son of Isildur. One of three who died with their father and many of their people during the Battle of the Gladden Fields. The other two brothers were Elendur and Ciryon. Their youngest brother, Valandil, was too young to go to War and so survived in Imladris and the line of Isildur endured.
Encyclopedia entry originally written by Nienna-of-the-Valar