Don't be a thickhead; read the instructions before you start the machine.

Even after the meeting, the thickheads refused to accept the new plan.

He was so thickheaded that he kept repeating the same mistake despite everyone's warnings.

Her thickheadedness prevented her from seeing the obvious solution until someone pointed it out.

Despite repeated warnings, he thickheadedly ignored the evacuation orders and stayed in his flooded house.