When in doubt... urbandictionary.com will usually give you a pretty good idea. Here is the top rated response for that question:
1. Beasters
371 up,
47 down
Known as the "Lows of the Highs" this type of marijuana is the brick weed of Canada. Grown in mass quantities in British Columbia this marijuana is usualy striped of keif, sprayed with chems, thrown into bags, and shipped over to the U.S. where people can pay up to 40$ for 3.5g. This marijuana is NOT low grade pot compared to most of the weed going around. Though the high may be not quite as good as that from headies like sensi star and northern lights, you tend to avoid the confused, head-ache high that commercial pot gives you. If commercial weed gives you anything at all.
I smoke beasters only when there are no headies around.