Meaning: To your good health. Something apparently unclean, yet benign in effect.
John 9:6-7
When he had thus spoken, he spat on the ground, and made clay of the spittle, and he anointed the eyes of the blind man with the clay, [7] And said unto him, Go, wash in the pool of Siloam, (which is by interpretation, Sent.) He went his way therefore, and washed, and came seeing.
"Here's mud in your eye!' said one of the modern pilgrims, tossing down his martini"
H.V. Morton, In Search of England (1927)