When sitting on the toilet in a public restroom. Someone who tries to look in to see what you are doing. Often but not always it's kids who are just curious. Peepweasels look under the door or partition, or stand outside and press their faces against the crack around the door to get the best view possible.
Man, I was taking a dump in the restroom at the mall lunchtime, and I noticed some little "peepweasel" looking at me through the crack in the door.