Albert Barnes Commentary Psalms 68:21

"But God will smite through the head of his enemies, The hairy scalp of such a one as goeth on still in his guiltiness." — Psalms 68:21 (ASV)

But God shall wound the head of His enemies - More properly, “God shall crush the head,” etc. The idea is that of complete destruction—as, if the head is crushed, life becomes extinct. See Genesis 3:15; compare Psalm 110:6.

And the hairy scalp - More literally, “the top of the hair.” The Hebrew word used here for “scalp” means the vertex, the top, the crown, as of the head, where the hair “divides itself”; and the idea is properly, “the dividing of the hair.” Gesenius, Lexicon. The allusion is to the top of the head; that is, the blow would descend on the top of the head, producing death.

Of such a one as goes on still in his trespasses - that is, the man who perseveres in a course of wickedness. If he repents, God will be merciful to him; if he persists in sin, he will be punished. The literal rendering would be, “the hairy scalp going on, or going (namely, ‘about’), in his trespasses.” The reference is to a wicked man continuing in his transgressions.