The Collaborative International Dictionary of English v.0.48:

Raca \Ra"ca\ (r[=a]"k[.a]), a. [Gr. "raka`, from Chaldee
   r[=e]k[=a].]
   A term of reproach used by the Jews of our Savior's time,
   meaning "worthless."
   [1913 Webster]

         Whosoever shall say to his brother, Raca, shall be in
         danger of the council.                   --Matt. v. 22.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Raca
   vain, empty, worthless, only found in Matt. 5:22. The Jews used
   it as a word of contempt. It is derived from a root meaning "to
   spit."


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

Raca, worthless; good-for-nothing