It comes from the 1994 film, The Crow. A character named Tin Tin (played by Laurence Mason) calls the sleazy pawnbroker Gideon a "chrome-dome child-molesting saprophyte". A saprophyte is a biological term for an organism that feeds on dead or decaying organic matter, such as certain fungi or bacteria. At the time the movie came out, I had worked mids for years and considered myself a mushroom because I never saw daylight and my job constantly gave me shit. I needed a gamer tag and it seemed a fitting moniker at the time.