The definition of a 'prickweed' is somebody who is an asshole, worthless and shouldn't exist. You know who you are. They also probably like smoking weed.
Low-quality marijuana, usually grown outdoors, with seeds, in Mexico. The Mexicans smuggle it to the US in highly-compressed bricks, hence the name. Brickweed only refers to the way it was packaged, and there can be such thing as good brickweed, but it's usually a bad thing.