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1.
To enter ones contents of the bowels in to the bog. i.e. have a fucking good shit
I'll be gone sometime im off to the shitter for a muck out.

Dont go in their it stinks i've just had a fucking good muck out.

Wheres the bog, i need a muck out.

Your fucking cat has just had a muck out all over the fucking floor.

Look at that dog its having a nasty muck out, and it fucking stinks.
by Cunts November 03, 2006
 
2.
To go for a shit. To deficate.
'Ive just had one hell of a Muckout'.

'Someones had a Muckout in there, it stinks they have proper smashed the fuck out of trap 1'.

MO = Muckout, 'Dont go in there ive just been for an MO'

'What a Muckout, someone better call a plumber'
by BenJ1111 January 03, 2010
 
3.
the transition between making out and fucking
Johnny Godeeper: "I made it to the muck out bro, but I didn't go all the way."
Melvin: "What's muck out?"
Johnny Godeeper: "It's like when you run from third to home...and the pitcher tags you out."
by esposed July 02, 2010