1 Samuel 25:17
Now therefore know and consider what thou wilt do; for evil is determined against our master, and against all his household: for he is such a son of Belial, that a man cannot speak to him. Now therefore know and consider what thou wilt do; for evil is determined against our master, and against all his household: for he is such a son of Belial, that a man cannot speak to him.
Immediately after Nabal insulted and dismissed David's messengers, one of his servants hurried to Abigail to warn her. The warning was that Nabal's insolence toward David had put Nabal and his whole household in immediate danger, because David was determined to take revenge and bring destruction upon them. The servant showed that abuse to be inexcusable and retaliation certain by testifying that David's men had been kind and harmless to Nabal's workers in the fields, with nothing lost and with the sheep guarded day and night like a wall.
Calling Nabal a son of Belial describes him as a thoroughly wicked, vile and selfish person. Saying that no man could speak to him means he was so hostile, harsh and unreasonable that he would not listen to counsel. Because he could not be reasoned with, the need for quick, decisive action to avert the harm fell on Abigail, who had to act at once without his consent. Dealing with such a vile person is harmful, so others avoid him in order not to be wounded by association.