Exodus 32:9

And the Lord said unto Moses, I have seen this people
He had observed their ways and works, their carriage and behaviour; he had seen them before this time; he knew from all eternity what they would be, that their neck would be as an iron sinew, and their brow brass; but now he saw that in fact which he before saw as future, and they proved to be the people he knew they would be; besides, this is said to give Moses the true character of them, which might be depended upon, since it was founded upon divine knowledge and observation:

and, behold, it is a stiffnecked people;
obstinate and self-willed, resolute in their own ways, and will not be reclaimed, inflexible and not subjected to the yoke of the divine law; a metaphor taken from such creatures as will not submit their necks or suffer the yoke or bridle to be put upon them, but draw back and slip away; or, as Aben Ezra thinks, to a man that goes on his way upon a run, and will not turn his neck to him that calls him, so disobedient and irreclaimable were these people.