In cannabis culture, shwag most often refers to low quality marijuana, generally grown in mass quantities. Shwag typically contains many seeds, which the user must remove, and has a somewhat lower THC content, meaning a larger amount must be smoked to feel high.

Based on the usage in cannabis culture, the term is now used in some contexts to describe things that are unpleasant, unwanted, or subpar.
http://encyclopedia.tfd.com/Shwag