1. To flush the toilet prematurely, or before one is finished "going" to the bathroom-- Usually used to mask sounds or when in a hurry.
2. Having to flush the toilet before one even begins to use the bathroom, due to prior droppings or toilet paper left behind by the previous user.
1. "Dude, I had diarrhea this morning and had to Pre-Flush so my sister's friends couldn't hear. I like my sister's friends."
2. "Ok, who didn't flush the toilet? I had to Pre-Flush a giant floater before i went, c'mon!"