I said unto the fools - To the wicked people in rebellion. Folly and wickedness in the Bible are synonymous terms, as they are identical in fact. See the notes at Psalms 14:1.
Deal not foolishly - Act not foolishly; carry not out your wicked plans. Do not pursue your schemes of wickedness and folly, for they cannot be successful, and they will only tend to involve you in ruin.
And to the wicked - The wicked people engaged in rebellion - either against a lawful human government, or against God.
Lift not up the horn - The horn is a symbol of strength. Compare Job 16:15; Daniel 7:7-8, Daniel 7:11, Daniel 7:21; Daniel 8:5, Daniel 8:8-9, Daniel 8:21. This is to be understood as the language of the person represented as speaking in the psalm - whether a prince, or whether God himself. It is counsel addressed to the wicked, that they should not attempt to put forth their strength in the accomplishment of their evil purposes. The reason given for this is stated in Psalms 75:6, namely, that success does not depend on chance, or on human power, but must come from God.