But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire. Matthew 5:22
" In Jesus’ day the word raca was used so much that it appears in the Greek NT in that form, untranslated. In both Hebrew and Aramaic its fundamental meaning was 'empty.' The human dignity of one’s neighbor should not be disparaged. The Hebrew verb 'to curse,' lekallel, comes from the root kal or “light” (in weight).
In practice the humiliation of another is tantamount to cursing him. There is no-one who is “empty-headed” and completely worthless. George Lamsa, who speaks Aramaic as his mother-tongue, points out that rekiah suggests derivation from the root rak which means in both Hebrew and Aramaic 'to spit'.”
Lamsa also wrote, "This word was not translated into Greek. Perhaps the translator could not find a similar habit of spitting among the Greek people as among Semites or he may not have known the meaning of the word. During heated arguments and controversies, easterners often spit in each other’s face. Merchants and prospective customers, after long bargaining and arguments concerning price, spit in each other’s face when they fail to agree. Nearly all quarrels are started by spitting. One often says raka arek na bapek, I will spit in your face."
"In the East utter contempt for someone is shown by spitting in their face...as was done to Jesus when he was subjected to the nocturnal interrogation. Worse than this, Jesus held, was to call someone a “fool.” This is a common insult amongst the Jews and the Arabs. Disagreements must always be settled while they are still fresh. They create an atmosphere in which, the Wise say, the Holy Spirit does not take pleasure."
(Santala, Risto. 1992. The Messiah in the New Testament. Jerusalem: Keren Avah Meshihit., pgs 168-169)