This toast is usually regarded as originating in Italy, where it is spelled "cin cin" (in Italian, "chin" would be pronounced "keen"). One explanation I've seen is that it derives from a Chinese expression of courtesy, "ching", literally meaning "I pray", said when a drink was offered, and misunderstood by early Italian sailors and travellers. Why the Italians especially should adopt this expression as a toast is not clear; and like many other explanations of the origins of words and phrases it may be fanciful.