Is it permissible to use food for the skin or body and wash it off inside the bathroom?

The basic principle is the obligation to respect food that is fit for consumption. It should not be used externally on the body except out of dire necessity or something close to it in terms of need. If its use becomes permissible and it is used, or if it spoils and is no longer fit for human or animal consumption, then there is no objection to disposing of it by washing it down the drain or throwing it out.