It's when you eat way too much fast food, probably McDonald's, and you get the runs.
Yo man, I got the McShits today from eating all that McDonald's.
The act of using the public restroom in a fast food restaurant without purchasing any food. When challenged by a suspicious manager, the assurance of a subsequent purchase of said fast food is known as a "McShit with Lies".
Dude, stop here, I can't hold it in anymore. I gotta go for a McShit. Meet me at the Drive-Thru in a few minutes.
Usually 30 minutes after the complete ingestion of a McDonald's meal one is faced with McCramps. These cramps are just like regular stomach pains, but differ in the fact that they are brought on by eating McDonalds.
Usually 10-15 minutes after the McCramps/McPains occur one will get the McFarts. These are generally farts with an odor similar to that of death. They reak and reak and generally cause people with you during that time to leave.
Finally, the McCramps get so bad that you run to the washroom and begin to expell feces. These feces are refered to as McShits. They are usually consist of diharea or some liquid and some solid parts. They also have an odor of death, and can cause one to pass out if they McShit too long. Generally, after a good round of McShits, the Large Intestine feels as if it has had an enema, and is generally happy until it one eats McDonald's again.
After the McShits subside you are done with the cycle, or your body may still be undergoing the the "McShit" process, and one will continue to McFart until the McCramps go away. That is the signal to enter society once again.
Matt: Omg Dude McDonald's is so good!
Sasa: I know, but it'll be bad later man, trust me...
Matt: How do you figure?
Sasa: The McShits bro, you'll see.
----- 30 Mins Later -----
Matt: Oh dude this is killing me! Ahh, what is this sensation?
Sasa: Thats the McCramps man... I told you!
Entering a fast food restaurant with the sole intention of using their toilets. If you tell a passing staff member that you'll buy their food afterwards, you have taken a McShit With Lies.
"Right... hold on people, I need to go for a McShit."
Using a fast food restaurant's facilities to relieve yourself when nature calls and not buying anything before you leave.
Wait her for a minute im goin for a Mc Shit.
The bad stomachache and diarrhea usually associated with eating too much Mcdonalds
Oh man those 3 Big Macs gave me a bad case of the McShits.
What you get after eating too much McDonalds food
Me - Mate, I've got the McShits
Random person - Oh yeah? Too much McDonalds food?
Me - Exactly