The Old Farmer’s Almanac had this:
Definition: A period of warm weather following a cold spell or a hard frost, Indian summer can occur between St. Martin’s Day (November 11) and November 20. Although there are differing dates for its occurrence, for more than 200 years the Almanac has adhered to the saying "If All Saints’ brings out winter, St. Martin’s brings out Indian summer." As for the origin of the term, some say it comes from the early Native Americans, who believed that the condition was caused by a warm wind sent from the court of their southwestern god, Cautantowwit.
Ask Yahoo! says this:
As the warm days of summer are replaced with the brisk nip of fall, your question proves the perfect puzzle for us to tackle.
According to USA Today, the term "Indian Summer" dates back to 18th-century United States. It can be defined as "any spell of warm, quiet, hazy weather that may occur in October or even early November." Other weather phenomena associated with a true Indian Summer include dry, hazy conditions and southwesterly winds that occur after the first killing frost.
The true origin of the term, however, is the subject of much research and debate. It first shows up in writing in 1778, in a letter from Frenchman-turned-American-farmer named St. John de Crevecoeur who wrote:
Sometimes the rain is followed by an interval of calm and warmth which is called the Indian Summer; its characteristics are a tranquil atmosphere and general smokiness.
Some speculate the weather phenomenon is named after the Native Americans because they were the first to recognize the pattern. They used the mild weather to hunt and harvest crops in preparation for the winter. Another theory suggests the name was inspired by the Native American belief that the favorable winds were a gift from a god in the desert Southwest. Still another possibility is that the weather pattern was so-named because it occurred more frequently out west, in "Indian" territory, than it did back east. And one rather prejudicial explanation suggests that in this context, "Indian" is used to mean false, so an "Indian Summer" is a false summer.
In a different vein, another theory suggests trading ships used this good weather period to travel to the Indian Ocean. Several ships actually had an "I.S." mark on their hulls at the load level thought safe during the Indian Summer.
Chances are, we will never know the origin of the phrase with any certainty.
And, finally, WordOrigins.org states the following:
Daniel J. Boorstin in his book The Americans, The Colonial Experience postulates an origin for the term Indian Summer, meaning a period of warm weather following the first frost of autumn. He states that European colonists endured raids by Indian war parties throughout the summer months. As the weather started to change, the raids would cease, so autumn was looked upon as a season of relative safety. If, however, warm weather continued into the autumn months, then this condition would mean another Indian Summer. A neat story, unfortunately as an explanation for the term’s origin, it’s quite wrong—proving that even a noted historian like Boorstin can be hoodwinked by popular etymology. (To give Boorstin some credit, his explanation is not totally without merit, having been first promulgated by Philip Doddridge in his 1824 Notes on the Indian Wars.)
Both Mencken and the OED2 fix the earliest recorded usage as 1778, rather late for fears of Indian raids. Even then, it was not a common term until well into the nineteenth century. It does not appear in Webster’s 1828 dictionary, nor in Pickering’s 1816 Vocabulary.
Several differing theories have been promulgated. The OED2 plumps for the idea that the phenomenon was more prevalent in the west, or Indian territory than on the Eastern seaboard. Mencken spends the most text describing the theory that it is called Indian Summer because it is a false summer. Indian is used in several contexts to denote something that is cheap or false, witness Indian-giver and Indian-corn.
Also supporting this idea is the British phrase St. Martin’s Summer referring to the same meteorological phenomenon. St. Martin suggests something cheap or false, primarily because dealers in cheap jewelry gathered at the location of London’s Church of St. Martin-de-Grand after it was torn down in the 16th century. St. Martin’s day is 11 November, which often corresponds to the spell of warm weather.
The first edition of Brewer’s lists a nautical term, Indian Summer Line. This term, which does not appear in the more recent editions, is probably unrelated. Under the entry for "Ship Letters," this book lists the following:
Ship Letters. These are to indicate when a ship is fully laden, and this depends on its destination.
* F.W. (Fresh Water line), i.e., it may be laden till this mark touches the water when loading in a fresh-water dock or river.
* I.S. (Indian Summer line). It was to be loaded to this point in the Indian seas in summer time.
* S. The summer draught in the Mediterranean.
* W. The winter draught in the Mediterranean.
* W.N.A. (Winter North Atlantic line).
-Tim
