doctrine of unclean hands
Legal doctrine describing the absolute wrongness of setting your Starbucks down on the urinal while you drain the weasel. By your act, the coffee is forever unclean, and any attempt to rationalize it that it was just the bottom of the coffee cup that touched the urinal is futile. Osmosis by grossness occurs, and you are basically drinking other dudes' urine.
One of the essays on the bar exam last year involved the doctrine of unclean hands.