What does the sergeant mean by 'Till he unseam'd him from the nave to the chaps and fix'd his head upon our battlements'?

Answer 1

The nave is the navel, and the chaps are the chops, the chin and jaw. If you have ever cut apart the seam which holds two parts of a piece of clothing together, you know what "unseamed" means--cut wide open. He cut Macdonweald wide open from his navel to his chin. Then he cut off his head and fixed it upon the battlements. The battlements are the outside wall of the castle. Fixed does not mean "repaired" here; it means "affixed" or "stuck". His head was stuck on the top of the wall.