File Off The Serial Numbers Meaning

(fandom slang, idiomatic) To remove the copyrighted elements from an existing work of fan fiction so that it may be commercially published as original fiction.

Example: 1998 March 6, Julie Lim, “Help!”[1], rec.arts.sf.composition, Usenet:
  For a while now, I've been pottering away at a Star Wars fanfic novella ([URL redacted], if you must know). I've received a few pokes about filing off the serial numbers and trying to sell it as an independent work.
2002 January 15, David Thornley, “Re: harry potter”[2], rec.games.int-fiction, Usenet:
  I've heard a rumor that Bujold started the Barrayar series as Star Trek fanfic, and there's enough resemblance there to make that seem very plausible to me. Suppose she did. By filing off the serial numbers, she was able to make the universe her own, to change as she needed to.
2012, Elizabeth Tai, "Quantity over quality?", The Star (Malaysia), 1 April 2012:
  I say “may have” because [E. L.] James has “filed off the serial numbers” of her fanfic.