The word Raca is an Aramaic expression of contempt, something like our English slang "stupid idiot!" Such an insult to a fellow believer in the Jewish dispensation might warrant being charged, before the council of the Sanhedrin, with slander.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8