Sentence:  He also had , at times , an uncanny absent-minded air like a sleepwalker ; he would look right through you while you were talking to him , and if you said , `` For Christ's sake , Donald , you 've got Prussian blue all over your shirt '' , he would smile , and <WSD>nod</WSD> , and an hour later the paint would be all over his pants as well . 
Meaning of the word marked with <WSD> tags:  lower and raise the head, as to indicate assent or agreement or confirmation