Easton's 1897 Bible Dictionary:

Ithamar
   palm isle, the fourth and youngest son of Aaron (1 Chr. 6:3). He
   was consecrated to the priesthood along with his brothers (Ex.
   6:23); and after the death of Nadab and Abihu, he and Eleazar
   alone discharged the functions of that office (Lev. 10:6, 12;
   Num. 3:4). He and his family occupied the position of common
   priest till the high priesthood passed into his family in the
   person of Eli (1 Kings 2:27), the reasons for which are not
   recorded. (See ZADOK.)


Hitchcock's Bible Names Dictionary (late 1800's):

Ithamar, island of the palm-tree