A derogatory expression used by Jews of the first century AD to show open contempt for another. Raca is derived from an Aramaic and Hebrew term meaning “empty” or “worthless.” Raca means "empty-headed." It likely insults one's intellect, not morals.
In the Old Testament, it is comparable to:
The worthless lot that Abimelech hired to follow him (Judges 9:4)
The idle men who gathered around Jephthah (Judges 11:3)
The scoundrels who joined Jeroboam (2 Chronicles 13:7)
Michal accused David of acting like a vulgar fellow [raca] who shamelessly exposed himself (2 Samuel 6:20). Rabbinic literature used this term to describe an immoral, untrained person.
Jesus warned against calling a brother “Raca!” (Matthew 5:22). Jesus said to judge the insulter and punish them with the highest penalty. The commandment against murder (Exodus 20:13) banned the act, angry thoughts, and contempt.