How did Hanun king of the Ammonites humiliate David's envoys?

Answer: Shaved off half their beards

Hanun shaved half of each man's beard and cut their garments at the hips. David told them to stay in Jericho until their beards grew.

“the princes of the Ammonites said to Hanun their lord, “Just because David has sent you comforters, do you really believe he is showing respect for your father? Has not David instead sent his servants to explore the city, spy it out, and overthrow it?””— 2 Samuel 10:3 (BSB)