Flushing a particularaly bad-smelling turd down the toilet in the stall of a public bathroom while you are still crapping so other people will not smell it. This can be repeated as many times as necessary.
Person in right stall: *Poops*

Person in left stall: "Oh God! That's Horrible! How about a
Mercy Flush?"

Person in right stall: *Flushes*

Person in left stall: "Thanks."

Person in right stall: "Welcome" *Continues Pooping*
by VegitarianZombie July 23, 2009
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The extra flush that comes between wiping and dumping, used when the shit is unusually large to prevent a plug of soggy TP and feces forming in the pipes.
"Shit guys, I took the biggest crap yesterday, I had to mercy flush so the toilet wouldn't plug!"
by Lo Ellen Knight 275 March 26, 2009
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when a dump smells so bad that you must flush half-way through for your own or the person in the next cubicle's health
omg this dump stinks, i better mercy flush
by Da Scrummage King May 4, 2010
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Flushing the toilet, as not to clog it if your taking a big crap

-- Used if you are taking such a big crap that it might clog the toilet, so you flush 1/2 way.
Man, give that thing a mercy flush, so it wont overflow!
by J. H August 27, 2005
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When you flush a dying fish down the toilet so he can have 'one last ride'
Ooh it looks like Goldy's headed for the big ocean in the sky...better give him a mercy flush.
by Dory March 30, 2004
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When Gary's shit smells so bad that he should flush the toilet before the turd even came out.
by Anonymous February 15, 2003
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