“Please do not let my lord.” Abigail addresses David in the third person out of respect.
“man of Belial.” This is a designation of sons of the Devil, although Abigail likely did not mean it that way; she would have meant it more as “worthless man,” or something like that.
[For more on sons of Belial, see commentary on 1 Samuel 2:12. For more on the unforgivable sin and children of the Devil, see commentary on Matthew 12:31.]
“Nabal.” The word “Nabal” means “fool,” but there are several different words for “fool” in Hebrew. The word “Nabal” refers to a committed, unrepentant fool.
“folly is with him.” This is a Hebrew idiom. Today we would say “He is a fool,” but in the Hebrew idiom, the phrase is “folly is with him” (cp. Job 12:13, 16).