The act of pooping in your pants while eating a McDonald's sandwich. Mcphailing can sometimes be accompanied with retching, regurgitation and in extreme cases, chain farting and/or bleeding.
sparkle had the worst mcphail in the history of the world. she crapped herself while eating a McRib
A Mcphail is someone who plays guitar in a band and loves iron maiden. They also have to have long black hair and play guitar. Also wear t shirts with no sleeves so you come across as hard.
Kern in badminton lessons when he plays guitar with badminton racket, whilst wearing a sleevless t shirt, and he has long black hair.
At that current moment, Kern is a mcphail.