“foolish plan.” The Hebrew text has the genitive, “plans (or ‘schemes’) of folly.” This is a beautiful double entendre. The genitive can be a genitive of relation, “the schemes that involve folly” (cp. ESV, NASB, NKJV), or a genitive of origin, “the schemes that come from folly” (Waltke). We have trouble bringing both concepts into English in one line except by just leaving the genitive in place as the NIV does. We need to be aware of the close relationship between foolishness and sin. Schemes that come from a foolish heart, and those that involve “foolishness” (which is related to the “stubborn fool,” the kesiyl (#03684 כְּסִיל), see Appendix 9; “Fool and Foolish”) are not “just fun,” they are sin, as this verse says.