The observation of a girl's underwear, hopefully a thong, stickin out of their pants. If it's in the back, we call 'er the regular 'peeker.' For the front, see frontal peeker
"Ooo shit man...look! look! look!...she's peekin"
"Wait for it....wait for it....PEEKER!"
"We got a lil' peekpeek I see"