"miming like Harpo Marx as they daren't shout out loud."

Although The Marx Brothers had their film heyday in the 1930s they were still popular in the 1950s. Harpo, the brother who played dumb (though in reality he could speak perfectly well) was a perfect foil to the voluble Groucho. Harpo got his points across through mime and the honking car horn that he wore at his belt.