Boris would go under the blinds to get to the 5 inch deep window sill until I put up this shelf. Then he could go right through the blinds instead of under them. Since I put up the shelf, maybe I'm the derp!
Wikipedia quotes the creators of South Park, the TV show. who say it is the sound commonly made in a klutzy situation when things go horribly wrong. They named a character, Mr Derp. If you are familiar with The Simpsons, it is similar to Homer Simpson's "Doh."