A Frederik Petersen defines someone who is like he is. It is a typically used definition of someone who is stupid, used in the area around Copenhagen, Denmark, but might not be known where you live, since it is slang. He tends to believe and try to convince other people that he is interested in many things and is an expert at things that he is not. Besides that he can be very funny, cool and likable but is special and kind of a know it all.
See that guy over there? He is such a Frederik Petersen.
I could bang a Frederik Petersen rn.