Speak not with a stiff neck - Mr. Bruce has observed that the Abyssinian kings have a horn on their diadem; and that the keeping it erect, or in a projecting form, makes them appear as if they had a stiff neck; and refers to this passage for the antiquity of the usage, and the appearance also.
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.”