“your horn.” The psalmist compares the boastful people to a wild ox that lifts up its horn against heaven and stretches its neck out in arrogance. This is the figure hypocatastasis, where the comparison between the boastful people is implied, but not stated [For more on the figure hypocatastasis, see commentary on Rev. 20:2]. The idea of the verse is, don’t be so proud that you would challenge heaven, and don’t be so arrogant.
“an arrogant neck.” The Hebrew, “a neck of arrogance,” means a stiff, hardened neck that will not turn direction and thus is not humble or reasonable.