While the definitive origin is lost to history, the phrase stands as a permanent marker of how early Americans interacted with, and interpreted, both the landscape and the indigenous peoples around them.
This theory suggests the name derives from early Native American tribes. The story goes that Native Americans would sometimes trick colonial settlers by pretending to call a truce or peace, only to attack when the settlers let their guard down. By analogy, this warm spell in autumn tricks nature—and people—into thinking summer has returned, before winter attacks in earnest. indian summer etymology
"Then the severe frosts begin, and are soon succeeded by a short season of mild, hazy weather, which is called the Indian Summer." While the definitive origin is lost to history,
Because no definitive record exists of the term's coinage, several competing theories have emerged: What is the origin of "Indian Summer" and is it offensive? By analogy, this warm spell in autumn tricks
It is worth noting that the phenomenon itself was not unique to North America, but the name stuck there. In Europe, similar warm spells are known as "St. Martin’s Summer" (Britain) or "Altweibersommer" (Old Women's Summer in Germany).