file off the serial number(s)

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. Someone filed off the serial number of the stolen car, so there's no way to track its original owner. Dude, you've gotta file off the serial numbers before you sell that gun. You don't want it to be traceable.

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. I know Fifty Shades of Grey began as Twilight fanfic, so I wonder at what point EL James filed off the serial numbers. He's good at writing fanfic, but don't have him file off the serial number—he's not capable of creating compelling characters from his own imagination.