McFail is used to describe failure on a large scale. It is used when the word fail or failure just won't cut it.
"Sean McFailed at nearly everything he did."
"I can't believe you did that, you're such a McFail!"
When Mc Donalds fucks up your order to the point of shooting up the place and shitting in the fry-cooker in an act of pure homocydal vengence
THERE'S PICKLE AND MAYO ALL OVER MY LARGE FRY! WHAT THE FUCK!? HOW DARE YOU GIVE ME A MCFAIL!!!!!!!!!!!!!!!
when u go large with something at mcdonalds and realise u can't eat all of it therefore looking like a twat
person1: im goin large
10 minutes later.....
person2: why did u go large??
person1: i shudn't have im full
person2: ur not gona eat that half portion of chips left
person2: haha mcfail!!!