The fantasy football industry is a booming, 24-hour business at this point. That being the case, when we use the word "sleeper," we understand now that it has a specific context within the nonstop world of fantasy football.
A fantasy "sleeper," in 2019, will rarely be a player no one has ever heard of. Sure, it happens sometimes—usually in season, when a third-string running back or an unestablished rookie breaks out for a ton of yards or a flashy score. Then everyone runs to the waiver wire.
But in the offseason, during your fantasy football draft, a sleeper is more commonly a player whose name might be familiar to you, but who people are sleeping on, in the sense that his Average Draft Position (ADP) is much lower than his upside.