S: (n) sleeper (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)
S: (n) sleeper (an unexpected achiever of success) "the winner was a true sleeper--no one expected him to get it"
S: (n) tie, railroad tie, crosstie, sleeper (one of the cross braces that support the rails on a railway track) "the British call a railroad tie a sleeper"