Matt Mitrione appeared on the tenth season of UFC’s The Ultimate Fighter. During the show his coach Rashad Evans referred to Mitrione as ‘Meathead’ and since then the nickname stuck.
However it wasn’t actually Evans that originally came up with the nickname. Mitrione explained that his “high school football defensive line coach couldn’t pronounce my last name well. One day he was upset with me, he was yelling at me and he couldn’t blurt out my last name. So it just came out, “Meathead”. So that’s how it came out.”