“A wholly different origin of the ‘Indian summer’ has been suggested by Mr. Horace E. Ware, who indicates that Indian in this connection may have referred to a nautical use in the British Indian seas.  Under the Regulations of the British Board of Trade one of the load-lines on ships bars the initials ‘.I.S.,’ thus indicating the maximum depth to which vessels can be loaded for voyages during the ‘Indian summer,’ which means the fine weather season in the Indian seas.  It is possible though unlikely, that the Indian summer of the eastern United States was named by travelers or seamen who saw in it some resemblance to the fine weather during the northeast monsoon of India.

Whatever may have been its origins, the term Indian summer was evidently used in the eastern United States, probably in New England…”

Reference Data:

Proceedings, American Philosophical Society, (Vol. 62, 1923), page 51