Sentence:  The wraith-like events of the last few days flooded David 's mind and he rubbed his unshaved chin and felt again the ache in his <WSD>kidneys</WSD> caused by his saddle odyssey from Boston . 
Meaning of the word marked with <WSD> tags:  either of two bean-shaped excretory organs that filter wastes (especially urea) from the blood and excrete them and water in urine