Condition in which a man with a "laxflow" and a headband who drives an Audi that smells like McDonalds falls in and out of love with anything on two legs that happens to have a baby maker.
Common Symptoms include laying on the couch talking about how said female is "the most beautiful girl in the world," eating chicken wings, and desperately trying to jump her bones at late night.
Definition stems from the number of infamous McGrath's who have matriculated through the St. Joseph's Preparatory School Rowing Program.
"Bro, I totally pulled a McGrath yesterday. But its okay because that girl is the most beautiful girl in the world."
"Remember when he McGrath'ed that girl? She broke up with him but its cool because she's the most beautiful girl in the world."
"Having McGrathism isnt a bad thing. Especially if she's the most beautiful girl in the word."