Lapdogs were all the rage among the new rich in America shortly after the Civil War, especially King Charles and Blenheim spaniels, rather imperious-looking dogs to the common man and certainly very distant relatives sociologically of the average American mutt, who had to work for his supper. These snooty dogs being pampered by their snooty owners probably inspired the expression putting on the dog, showing off, which apparently arose in the 1860s as college slang at Yale University. Attempts to derive put on the dog from the older put on side all seem strained. The reasoning behind the latter is that dogs “show off” by arching out one side while moving their feet in intricate maneuvers.