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1.
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?"
by lespoos June 26, 2010
 
2.
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..."
by naomi.j August 31, 2008