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.

The thoughts of foolishness is sin,.... The thought of sin is sin (e), before it comes into action; the motions of sin in the mind, the workings of corrupt nature in the heart, the sinful desires of the flesh and of the mind: these are forbidden and condemned by the law of God as sin, which says, "Thou shall not covet", Exodus 20:17, and stand in need of pardoning grace and mercy; see Romans 7:5. Or, "the thoughts of a foolish man are sin" (f); that is, of a wicked man; in all whose thoughts God is not, but sin is; the imagination of the thoughts of his heart is evil, and that continually; he thinks of nothing else but sin, Genesis 6:5;
and the scorner is an abomination to men; who not only thinks ill of divine things, and despises them in his heart, which is only known to God; but scoffs at them with his lips, makes a jest of all that is good, derides religion and religious men; and to such he is an abomination: and indeed one that is proud and haughty, scorner is his name, and that deals in proud wrath, and scorns all around him, in whatsoever company he comes, and that ridicules every person, and every thing that is said in conversation, is usually hated and abhorred by all sorts of men.
(e) "Nam scelus intra se tacitum qui cogitat ullum, facti crimen habet", Juvenal. Satyr. 13. v. 209, 210. (f) "stulti", Pagninus, Junius & Tremellius, Mercerus, Piscator, Gejerus.

Same thought varied.

This proverb is connected by זמת with Proverbs 24:8, and by אויל with Proverbs 24:7; it places the fool and the mocker over against one another.
The undertaking of folly is sin;
And an abomination to men is the scorner.
Since it is certain that for 9b the subject is "the scorner," so also "sin" is to be regarded as the subject of 9a. The special meaning flagitium, as Proverbs 21:27, זמּה will then not have here, but it derives it from the root-idea "to contrive, imagine," and signifies first only the collection and forthputting of the thoughts towards a definite end (Job 17:11), particularly the refined preparation, the contrivance of a sinful act. In a similar way we speak of a sinful beginning or undertaking. But if one regards sin in itself, or in its consequences, it is always a contrivance or desire of folly (gen. subjecti), or: one that bears on itself (gen. qualitatis) the character of folly; for it disturbs and destroys the relation of man to God and man, and rests, as Socrates in Plato says, on a false calculation. And the mocker (the mocker at religion and virtue) is תועבת לאדם. The form of combination stands here before a word with ל, as at Job 18:2; Job 24:5, and frequently. but why does not the poet say directly תועבת אדם? Perhaps to leave room for the double sense, that the mocker is not only an abomination to men, viz., to the better disposed; but also, for he makes others err as to their faith, and draws them into his frivolous thoughts, becomes to them a cause of abomination, i.e., of such conduct and of such thoughts as are an abomination before God (Proverbs 15:9, Proverbs 15:26).

The thought - The very inward thought or contrivance of evil, is a sin in God's sight.