Coating the Penny Horse

This British phrase was originally to describe the act of painting a fresh coat of paint on a small china horse, often found in bric-a-brac / jumble sales and charity shops. The phrase is now primarily used to describe going for a messy poo on a cheap toilet.

"Our John's been busy coating the penny horse this morning."

"By gum, that curry last night were right dodge -- I've got to coat the penny horse, I'll be back in half-hour."