Albert Barnes Commentary Psalms 75:5

"Lift not up your horn on high; Speak not with a stiff neck." — Psalms 75:5 (ASV)

Lift not up your horn on high - In a proud, self-confident, arrogant manner.

Speak not with a stiff neck - With arrogance and pride; in a haughty, imperious manner. The word rendered “stiff” (literally “a neck of stiffness”) - עתק ‛âthâq - means properly bold, impudent, wicked; and the idea is that of speaking as those do who are impudent, shameless, bold, licentious - indicating confidence in themselves, and a reckless disregard of truth and of the rights of others. The Septuagint and the Vulgate render it, And speak not unrighteousness against God.