The Milk Leaver is a person who desires to use the milk in the fridge, but discovers, when they lift up the carton, that it is about to run out.
So, instead of actually using up all of the milk and being forced to take responsibility for it. (having to fold it up, throw it in the garbage, locate more milk and open it) this sad example of humanity simply leaves a tiny amount of milk in the carton or bag, not even enough for a squirt in a small cup of coffee. This neatly passes the problem onto somebody else and allows them to use the “Who, me? But there was milk left!” defense with a clear conscience
Guy1: There's barely any milk in this bag
Guy2: It must have been a milk leaver