Lick might be a weird slang in rap, but it is actually commonly used, especially by the somewhat older generation of rappers.
The term was actually popularized by Scarface in 1991. When he first used it, he primarily referred to a robbery or robbing someone to quickly acquire large amounts of cash.
“If this big lick comes through, we are going to be rich!”
However, some have used it to refer to hitting the jackpot in gambling:
“You better run when you lick in a casino.”
Lastly, when used as a noun or description of a person, “Lick” might mean someone that gets lots of cash quickly, primarily through illegal means:
“You better stop dating that lick, or you might get into trouble!”