1) empty, i.e. a senseless, empty headed man
2) a term of reproach used among the Jews in the time of Christ
Part of Speech: noun
Of Chaldee origin (compare [H7386]); O empty one, that is, thou worthless (as a term of utter vilification): - Raca.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root ῥακά; click on the second column to search for that grammatical form of the root ῥακά; click on the third column to search for that word and grammatical form; click on the total to search for the root ῥακά.