According to Bon Appétit, “some claim that it was named for ‘grinders,’ Italian-American slang for dockworkers (who were often sanding and grinding rusty hulls to repaint them),” but the term most likely comes from the fact that they were harder to chew than normal sandwiches: “that toothsomeness got translated into ‘grinder,’ since that’s what your teeth had to do to get through a bite.”

"Grinder" shares some flimsy nautical roots with the sub--some claim that it was named for "grinders," Italian-American slang for dockworkers (who were often sanding and grinding rusty hulls to repaint them)--but the more widely attested origin is about the sandiwch itself.