A vehicle, usually an import tuner, that is heavily modified under the hood (built engine, forced induction etc), capable of being faster than most other modified cars, however has non-performance damaging flamboyant cosmetic modifications (stickers, a large spoiler, hood/roof scoops, large obnoxious muffler).
A ricer-sleeper accomplishes being a sleeper (a car that does not look fast but really is) by appearing to be a ricer (a car that is modified to look cool and fast but in reality is not).
Guy 1: OMG! LOOK AT THAT RICER! Looks like he took the Fast and Furious trend too far...
Guy 2: LAWL YES, i bet it's a stock, base model, automatic!
Guy 3: That thing just beat my 300HP Nissan 5 minutes ago...
Guy1: OMGAWD its a sleeper??
Guy2: OH SHIZ! It's a ricer-sleeper.