The character sequence "Peh" is an onomatopoeia which means that it signifies a noise that one would emit. This noise would be the act of spitting to the ground (as if spitting on the forementioned subject of verbal debate). The other sound it implies is an quick exasperated emission of air past the lips in disbelief or annoyance, the latter is usually accompanied by squinting eyes at the subject and shrugging one's shoulders slightly.
Freshman:"Dude, this biology homework is such total crap. wtf is wrong with our fat teacher?!"
Mother:"Go to bed right now, you aren't doing homework your IMing!
Son: "Peh?!...wtf *sigh*"