Brick is a slang term used to refer to any drug weighing a pound or kilogram. The term brick is mostly used for cocaine but it can be used for any drug packaged up and amounting to a pound or kilo. A brick often consists of pure drugs that haven't been stepped on.
Brick Example
In Stitches' "Brick In Yo Face" Song, he says "I put that brick in your face - Now what you gon' do with it?" Stitches is basically saying he's fronting you a kilo of coke, what are you gonna do with it? It's a rhetorical question since the individual that's getting the drug has to pay Stitches back, so he's clearly gonna break it up and sell it.