Sentence:  You have to hand it to Curtis Joseph. Not only does he keep pucks out of the net, he keeps secrets too. "It's taboo," said Joseph, grinning like the <WSD>cat that ate the canary</WSD>. "You're not going to get anything." 
Meaning of the word marked with <WSD> tags:  A person who appears self-satisfied or smug, especially while concealing something mischievous, prohibited, or private.