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3.
To flush the water-closet prior to defication thereby lubricating the porcelain, thus mitigating the effects of a crash landing.
'Dont forget to pre-flush darling, you do know how you love to crash-land'.
by McGinty September 02, 2006
 
1.
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!"
by Americanhockey1 February 03, 2011
 
2.
To flush the toilet before finishing your 'business'. Usually done when in a hurry. eg. During a movie.
Kevin : Wow, that was quick!
Dave : I Pre-Flushed so I didn't miss any of the film
by UltimateBurrito May 17, 2010