The Hebrew noun נבל (nāvāl, pronounced na-VAHL, with the medial letter "bet" pronounced as a "v"), means "fool" or "futile, worthless, good-for-nothing." It is derived from the verb of the same spelling, נבל (nāval), meaning "to be futile, foolish," as in Proverbs 30:32: "If you have been foolish (נבלת), exalting yourself, or if you have been devising evil, put your hand on your mouth."

There are many things that one can do to be considered foolish in the Bible. Perhaps understandably, not believing in God is considered foolish in Psalm 14:1: "Fools (נבל) say in their hearts, 'There is no God'." In fact, the entire nation of Israel is considered foolish when they do not follow God's law to the fullest, as in Deuteronomy 32:6: "Do you thus repay the LORD, O foolish (נבל) and senseless people? Is not he your father, who created you, who made you and established you?" And in a cringeworthy Job 2:10, Job calls his wife foolish, and is apparently not considered "sinful" for doing so: "But he said to her, 'You speak as any foolish woman (הנבלות) would speak. Shall we receive the good at the hand of God, and not receive the bad?' In all this Job did not sin with his lips."

There is also a man named Nabal in 1 Samuel 25:25, whose very name means "fool": "My lord, do not take seriously this ill-natured fellow, Nabal (נבל); for as his name is, so is he; Nabal (נבל) is his name, and folly (נבל) is with him; but I, your servant, did not see the young men of my lord, whom you sent."