In school, Matthew's friends called him "MatPat," a combination of the first three letters of his first and last names ("Matthew" and "Patrick" respectively).
Other nicknames and aliases
FatPat: In HOW MANY Calories is Stay Puft Marshmallow Man?, MatPat reveals an alternate personality, one obsessed with sugar and food. FatPat is willing to consume the entire Stay Puft Marshmallow Man, even if it kills him.
Matrick Patthew: A play on the rhyming of "Mat" in Matthew and "Pat" in Patrick, done by switching the second syllables of each name. Stephanie playfully exclaimed this name in Is MatPat ALIVE?! while Matt was explaining how his nickname works in response to the Google search suggestion "matpat's real name".