A term of exclamation used to describe a person, in reaction to something silly and/or humourous they say.
Not necessarily used as an insult, this term is most commonly used as a term of endearment and should never be taken seriously.
Taken from a combination of the words freak and prick.
Friend 1: I didn't know that the buffalo has wings...
Friend 2: It doesn't! You're such a phreak!