Sorry, local slang. I don't know any other way to describe it so I will try to explain. I am sure everyone is aware, I don't mean to offend, but I have no idea how to explain it any other way. So here it goes...
When an animal is frightened, running or being chased, the flood of hormones such as adrenalin, uric acid, etc, builds up under the hide. This also happens if the shot was not well placed and the animal experiences a slow death, or struggle. The longer the hide stays on the animal, the more of the hormones that the meat will absorb. This is what gives wild game that 'livery'