A stiff-necked people. Obstinate and self-willed. In the Psalms we read, “Speak not with a stiff neck” (lxxv. 5); and in Jeremiah xvii. 23, “They obeyed not, but made their necks stiff;” and Isaiah (xlviii. 4) says, “Thy neck is an iron sinew.” The allusion is to a wilful horse, ox, or ass, which will not answer to the reins.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894