1. Literally, to erase or make illegible the serial number of a registered product, especially a firearm or motor vehicle, by filing it with a metal instrument, so as to make it harder to trace when sold or traded illegally. Even with improved laws in place to help police track down stolen guns, many criminals are still able to distribute them on the black market by filing off the serial numbers.

2. By extension, to remove copyrighted elements from a fan-written piece of fiction ("fanfic") so that it may be sold or distributed as an original piece of work. I used the sci-fi series as a foundation for my fanfic, but—after many requests from readers—I have decided to file off the serial numbers and develop it as a standalone body of work.