The act of checking out a girl (or guy) and having a sudden vocal reaction (a la Quagmire from Family Guy). Typically the vocal response is something along the lines of, " Oh my god!" or "Holy Sh*&", and may be followed by such creepy actions as stalking or taking a photograph without the consent of the "quagee".
Friend 1: Dude! That girl is wearing nothing but lingerie (bites index finger).
Friend 2: Careful, you're about to quag out all over the floor.