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When you shoot cum on someone's back like it's the night sky, usually during a group fuck where everyone's trying to out-splash each other.
I just got galaxy back'd by three guys at the same time. My shirt is now a constellation map.
He said it was a surprise galaxy back. It wasn't. It was a setup.
She didn't even flinch when I did the galaxy back. She just said, 'Do it again, I like it.'