Sentence:  And then again perhaps the reason why he could n't find time to do any of the things he had planned to do after retirement : reading , roaming , gardening , lying on his <WSD>back</WSD> and watching the clouds go by , was because he did n't want to do them . 
Meaning of the word marked with <WSD> tags:  the posterior part of a human (or animal) body from the neck to the end of the spine