Where does the drinking toast "here's mud in your eye" originate?

It comes from the trench warfare of World War I, and the implication was that it would be good fortune only to get mud from the trenches in your eye, all the other very real possibilities, such as artillery shrapnel, being absolutely horrific.

It may come as a surprise but in all truth mud also existed in Vietnam. The picture on the left is of a major supply road inside the firebase named C-2.