sleeper

noun ! Definition from WordNet 3.1, Princeton University. Pronunciation from Wiktionary, via Kaikki.org. More about sleeper at Wordnik → Where our words come from →

1. A rester who is sleeping.
2. A spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal.
3. An unexpected achiever of success.

   the winner was a true sleeper--no one expected him to get it