empty-head: a casual term of contempt for someone treated as witless and worthless
Rhaka is not really a Greek word at all. It is an Aramaic insult that Matthew kept in its original sound rather than translating, the way he keeps a few other everyday Aramaic expressions Jesus used. It comes from a word meaning "empty," (ריק) and as an insult it means roughly "empty-head," "blockhead," or "good-for-nothing." To call someone this was to write them off as having nothing upstairs and nothing to offer.
In the world Jesus spoke into, this was a common, almost throwaway insult. Jewish teaching of the period is full of it: it shows up as banter between scholars, as a sharp word thrown at someone who has given offense, and occasionally as a deadly serious condemnation, depending entirely on who said it to whom and why. Lexicons often label it "mild," and at the level of everyday speech that is fair. It was the kind of dismissive jab people tossed off without thinking.
That ordinary, harmless feel is exactly the point in Jesus' one use of the word. In the Sermon on the Mount he traces murder back to the contempt behind it, and he puts this casual slur on the same footing as killing. The shock is not that the word is unusually vicious, but that something so common and so easily excused is named alongside murder before God's judgment. Calling a brother an empty-head treats a person made in God's image as a nobody, and Jesus refuses to let that pass as harmless.
Whether this insult is meant as a notch milder than the harsher "fool" (the word that follows it in the same verse μωρός) or simply names the same contempt from another angle is debated among scholars; see the disputes below.