Adam Clarke on Proverbs 24:9

The thought of foolishness is sin - זמת אולת חטאת zimmath ivveleth chattath. "The device of folly is transgression;" or, "an evil purpose is sinful;" or, perhaps more literally, "the device of the foolish is sin." It has been variously understood by the versions. "The cunning: of the fool is sin." - Targum. "The imprudent man (or fool, αφρων) shall die in sins." - Septuagint.

So the Arabic.

The thinkynge of the fool is synne - Old MS. Bible.

Fool is here taken for a wicked man, who is not only evil in his actions, but every thought of his heart is evil, and that continually. A simple thought about foolishness, or about sin itself, is not sinful; it is the purpose or device, the harbouring evil thoughts, and devising how to sin, that is criminal.