Sentence:  “Then what is your little trouble?” “My little trouble!” I felt that this sort of thing must be stopped at its source. It was only ten minutes to dressing-for-dinner time, and we could go on along these lines for hours. “Listen, old crumpet,” I said patiently. “Make up your mind whether you are my old friend Reginald Herring or an <WSD>echo</WSD> in the Swiss mountains. If you're simply going to repeat every word I say –” 
Meaning of the word marked with <WSD> tags:  A reflected sound that is heard again by its initial observer.