In short and simple terms, a "sleeper pick" refers to a relatively unknown or underestimated person or thing that turns out to be unexpectedly successful or impressive.
Sentences with «sleeper pick»
I wish I had more time to through all these kind of sleeper picks, it's so much fun. (sbnation.com)
I think he's a nice sleeper pick to score first, but shop around the get the best odds possible. (sportsinsights.com)