A term used to describe a large sweat patch that has formed on the back of a person's shirt/T-shirt.
Girl: "Man I'm so sweaty, do I have a Back Splat?"
Best Friend: "No no your fine..."
Poo that's been left on the back of the toilet after a dump. The shit stains left after a messy poo in the loo.
"I left a particularly messy back splat in the upstairs loo for the misses to clean."
"Who couldn't be bothered to clean the toilet after using it and left that nasty back splat?"