It derives from the background music on old Porn flicks. Its similar to bow chica wah wah but shorter and to the point. It originated in Amherst NY in the late 90's by a group of friends in High school who would use this sound to alert another friend that there is an attractive female in the general area. It's other use is to poke fun at at a male friend for showing affection towards a female.
Ron and Matt are engaged in a conversation. Ron looks up and sees an attractive female walking towards them. Ron then says Pewr Tewr and Matt looks over his shoulder and shakes his head in agreement.
Kevin and Kim are in the stairwell holding hands. From the floor above Tom shouts down the stairs Pewr Tewr in an obnoxious tone. Kevin, now embarrassed looks up to see his buddies laughing and pointing at him.