E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.