The origins of the phrase are uncertain but it did originate in the United States, and refers to the indigenous Native Americans, or 'Indians'. The explanation that I like the best, and I think fits the best, is that it refers to a warmer and milder than normal autumn, and was a favorite time for hunting. It meant the tribe could stay longer in their summer hunting grounds, which meant more hunting and gathering to get them through the winter.
No matter the origins, there is nothing more beautiful than an Indian Summer day, especially in Oxford which is about as far removed from Indian hunting grounds as you can get. The golden light bouncing off the golden stone never ceases to amaze, and like the American writer Nathaniel Hawthorne, I don't want to waste of a second of this lingering autumn sunshine.