Sentence:  A waiter brought his aperitif, which was a small scotch and soda, and as he sipped it gratefully he <WSD>sighed</WSD>./     ‘Civilized,' he said to Mr. Campion. ‘Humanizing.' […] ‘Cigars and summer days and women in big hats with swansdown face-powder, that's what it reminds me of.' 
Meaning of the word marked with <WSD> tags:  To inhale a larger quantity of air than usual, and immediately expel it; to make a deep single audible respiration, especially as the result or involuntary expression of fatigue, exhaustion, grief, sorrow, frustration, or the like.