n. when you're taking a huge dump and some of the liquid in the toilet splashes back up onto your ass
Dude, that burrito I ate came back to haunt me, there was toilet backwash everywhere. On my ass, in my ass, in came out of the toilet and onto the ground.
Backwash is often created inadvertently or unintentionally when liquid escapes the mouth during the process of drinking, though it can also be created intentionally by an act of spitting or purposely letting liquid fall from one's mouth and back into the container.
When multiple people drink from the same container, there will usually be some amount of backwash put back into the container; as the number of sips increases, in general, the concentration of backwash also increases.
"Dude, I don't want your backwash." (This person has perhaps declined someone's offer of their nearly empty can of Coke).
The act of Oral sex. When the girl from the decent of a redneck gets tired of her breath smelling like hick that she uses some listerine. She then gives another redneck great oral sex. The man ejaculates into her mouth, she spits everything into a cup, and the man then drinks it.