Top Definition
When you are on the verge of taking possibly the most disgustingly smelly, nastiest, painful, room clearing, eye watering, breath shortening, leg falling asleep shit ever...period.

Usually performed at the most inopportune time imaginable:
1) The company picnic where toilet paper is not accessable.
2)Your new girlfriends parents house when meeting them for the first time.
3)While driving the hottest chick home from the bar.
4)At a funeral.
5)At a wedding
6)In church
7)At a sorority house while in the midst of strip poker
8)Before or during a threesome with twins.
9)When cuffed in a police car, or in jail.
10)While attending a sporting event with "open stall" bathrooms.
11)at any store or restaurant without locks on the single throne bathroom.
12)At your inlaws.
A shat mcnasty is the shat of all shats, the dump of all dumps, the king of all craps, the morning after eating at any mexican taco stand.
Dude: "what happened last night after I had to leave?"

Dude 2: "the Olsen twins lost their hand in strip poker and got naked looking for you....where did you go?"

Dude: "after eating all that mexican food and drinking that pisswater they call beer, I went to the ihop and took a shat mcnasty. I blew it out in there and cleared the place, I think they called the fire department."

Dude 2: "did you go home and shower after that?"

Dude: "I had to"
by rileyriley April 29, 2010

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