This term for a period of warm weather after the first frost of autumn dates to 1778. From a quote by St. John De Crevecoeur in that year:
It [sc. snow] is often preceded by a short interval of smoke and mildness, called the Indian Summer.
No one is exactly sure why this period is called Indian summer, although there is no shortage of explanations, none with any real evidence to support them.
One of the earliest explanations was suggested in 1824 by Philip Doddridge in his Notes on the Indian Wars:
The smokey time commenced and lasted for a considerable number of days. This was the Indian summer, because it afforded the Indians another opportunity of visiting the settlements with their destructive warfare.
Another explanation is that Indian is used in a derogatory fashion in many compounds to denote something that is false or a poor imitation. This explanation is paralleled by the term St. Martin’s Summer, which is what the phenomenon is known as in Europe. It is called this because the warm spell often occurs around St. Martin’s day, 11 November. But in England, St. Martin is also associated with deception and falsehood because of the dealers in cheap jewelry who frequented the parish of St. Martin-de-Grand in London. This association may have reinforced that of Indian Summer being a false summer.
Copyright 1997-2013, by David Wilton