The act of extracting heroin-laden blood, for reinjection at a later time. It usually is one's own blood, but could also be the blood of someone who has just overdosed. (Note: NOT RECOMMENDED). It is done as "insurance" in case one's heroin supply runs out. The term is used in Neil Young's 'The Needle and the Damage Done': "milk-blood to keep from runnin' out"
I better milk-blood in case my dope runs out.