"A handsome and harmless serpent" [Century Dictionary], one of the larger snakes of the U.S., common in many states, by 1812, from milk (n.) + snake (n.). Also called chicken-snake (attested by 1793), house-snake, and thunder-and-lightning snake.
It [the milk-snake] sometimes in this county has been known to enter a grist-mill and remain a length of time for the apparent purpose of feeding on the mice which were there attainable. It is probable this is one principal object of his frequenting dwelling houses, and not always for the purpose of obtaining milk, as is generally supposed. [The American Journal of Science and Arts, April 1844]