This is one that I thought of today when I was doing the dishes.
monkey dish - a small dish used for restaurant sides. The two lines indicate the amount of food. The bottom line is at approx. 1/4-1/3 cup and the top rim line is approx. 1/2-2/3 cup. The lines were important for food workers to be able to judge portion sizes quickly without actually measuring.
Some people believe that the term originated in the homes of important people that actually used live monkeys to test their food for poison.
Another theory is that it came about with the Italian American organ grinders that would ask diners/restaurants for a small portion of food for their monkey. A portion similar in size to the little tin cup that the monkey used to beg for change.