The Sebasticook, better written as Sebesticook or Sebestiguk,
was a section of one of the most important travel-routes of
ancient times, the shortest way of going via Arnold’s Trail from the Penobscot to Quebec.  Though not a translation of the word, the best English equivalent is “The Shortest Route”.

Fanny Hardy Eckstorm , Indian Place-Names of the Penobscot Valley and Maine Coast p.11

The early history of the river is well depicted in this  quote.

“While the inhabitants seem to have found it better fishing in the Sebasticook rather than the Kennebec River, this may have been due to two factors: first, the river could be spanned easily by weirs and, second, the town was astride the river. Thus, the voters could control the fishing industry. This was not possible on the Kennebec, for Fairfield had possession of the west bank…”

HISTORY OF ALEWIVES IN THE SEBASTICOOK RIVER, MAINE
 Source: Fisher, Carleton Edward. 1970. History of Clinton, Maine. Kennebec Journal Press. Augusta, Maine.

Since that early part of the European settlement the seventy six miles of the Sebasticook River have been dammed, bridged, and diverted. It was a well used highway for the Abenaki and the French traders. It has provided food and energy for early settlers. It has fueled the growth of the nation in woolens, leather, shoes, lumber, and paper. Now it has the opportunity to return to itself and be whole again, to serve as our river.