sleeper hit

plural sleeper hits

1. A work of entertainment that becomes successful only some time after its launch.

   The record would be a minor hit in the U.S. (#49) but would, in time, become one of the most familiar songs of the decade. A 'sleeper' hit. It would not touch the UK charts at all.

   2005, George Gimarc, Punk Diary: The Ultimate Trainspotter's Guide To Underground Rock, 1970-1982

   1 more example

   It could be argued that there is no such thing as a 'sleeper' hit. It is industry terminology for a film which becomes a big success, despite having seemingly little promotion, and which plays successfully for a long time period, without having a record-breaking opening, or a run at the top of the box office chart.

   2008, John Berra, Declarations of independence : american cinema and the partiality of independent production