What does "the sleeper of the vintage" mean?

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Dear Dr. Vinny,
What does "the sleeper of the vintage" mean?
—Albert, Seoul, Korea
Dear Albert,
The term "sleeper" means an unexpected success. I've seen it refer to both wines and entire vintages. If it's a "sleeper of the vintage," it means this wine did surprisingly better than its peers. If it's a "sleeper vintage," it means that the vintage was considered unimpressive at first, but has since been recognized as better than expected.
—Dr. Vinny