Crap comes from Middle English crappe (chaff, waste), from Old French crappe, not from the Victorian plumber Thomas Crapper, whose name is a spectacular coincidence.
Excrement; also nonsense, rubbish, or something of poor quality.
From Middle English crappe, meaning chaff or residue from rendered fat. Probably from Old French crappe, meaning waste or rejected matter, which may derive from Medieval Latin crappa. The common belief that it comes from Thomas Crapper, the Victorian plumber, is a folk etymology — the word predates him by centuries. Key roots: crappa (Medieval Latin: "chaff