'Ivveleth (אִוֶּלֶת, Strong's #200), “foolishness; stupidity.” This noun appears 25 times in the Old Testament. It can mean “foolishness” in the sense of violating God’s law, or “sin” (Ps. 38:5). The word also describes the activities and life-style of the man who ignores the instructions of wisdom (Prov. 5:23). In another nuance, the noun means “thoughtless.” Hence 'ivveleth describes the way a young person is prone to act (Prov. 22:15) and the way any fool or stupid person chatters (Prov. 15:2).
Nebâlâh (נְבָלָה, Strong's #5039), “foolishness; senselessness; impropriety; stupidity.” This abstract noun appears 13 times in the Old Testament. Its use in 1 Sam. 25:25 signifies “disregarding God’s will.” Nebâlâh is most often used as a word for a serious sin (Gen. 34:7—the first occurrence).