Wet in the Paint has been used by bay-area rapper E-40 and is similar to the popular rap phrase "Goin hard in the Paint"; a reference to Basketball - the paint being the painted "key" around the basket. So the paint part is defined.
But wet.? Wet is slang for using PCP, or sherm.
So being, or going "wet in the paint" would be a step up from "going hard in the paint" wet in the paint would be more violent, drug-addled and careless. But still synonymous to gettn buck, goin hard, and goin dumb. Wet in the paint; think of Kobe Bryant on PCP=unstoppable.
E-40 - "Trained to Go" "I'm wet in the paint; I'm trained to go."