A common abbreviation for Mimosa Hostilis Root Bark. MHRB contains DMT; the most potent hallucinogenic drug known to man.
Since DMT is rarely sold, most psychonauts tend to purchase MHRB online and then extract DMT from it themselves using lye (drain/oven cleaner) and naptha (zippo lighter fluid), among other materials.
MHRB is one of the thousands of plants/animals that naturally produce DMT (including the brain of human beings).
Credit:
Urban Dictionary