The phrase "sleeping lion" refers to someone or something that is calm and peaceful at the moment, but has the potential to become strong, fierce, and dangerous when awakened. Full definition
The Arsenal I know will tear Man City apart yday but played like sleeping lions & managed to take their chances. (justarsenal.com)
August 18: Electronic duo Sleeping Lion and psychedelic folk rock band iives will each perform a set of music at the ICA (Boston) (bostonmamas.com)
«There is a risk building up in this regard, and on top of that, the tricky issue is that there are many triggers that could wake up this sleeping lion,» said David Daokui Li, a prominent Tsinghua University economist who is a member of the Chinese central bank's monetary policy committee. (nytimes.com)