Referring to a car, it is a car whose cosmetics look like it's stock and slow and one assumes there is a small engine under the hood. Only when it is challenged by a ricer or another car, it shows its mighty by leaving the other car in a cloud of smoke because it has either a huge engine from the factory or has a swapped engine from a bigger car or has tons of engine modifications.
A sleeper is is the perfect example of this acronym "All bite, no bark."
It is a sleeper dude, that is what it meant to do, shock you by its speed and power you never thought it had.
The exact opposite of a ricer. A Sleeper looks stock but is heavily modified to preform better. Sleepers are often, like ricers, economy cars. Unlike ricers, sleepers can also be regualr sedans or any other vehicle that was not meant to be a preformance vehicle. Sleeper cars are ussually modified with boost (super or turbochargers), transmisions, and sometimes full engine swaps. Many people who drive sleepers use them to hustle at the strip or avoid attention from the cops. Many auto makers offer factory sleepers that are a sport version of the origional economy sedan with very few external changes. Some of the best sleepers of all time came from the muscle car era. The 1972 Chevy Nova is considered one of the best sleepers. The Nova is one of the most popular models to get turned into a sleeper. Most sleepers aren't true sleepers because they will have aftermarker gauges visible (real sleepers hide them in the glove box), racing slicks and loud pipes. A true sleeper will not be exposed until they reach the end of the track.
Chuck's 1968 Mustang GT500 got spanked by a 1960 Chevy Nomad Wagon. Chuck was the victim of a big time Sleeper when the Nomad lifted the front wheels in the air and he proceeded to see nothing but tail lights down the track.