“Another geographical name found in the Paugasuck deed is "Quassapaug"—applied to the beautiful lake which lies just west of the western boundary of Mattatuck, part of it in Middlebury and part in Woodbury. In a Woodbury deed of October 30, 1687, it is spoken of as "the pond called and commonly known by the name Quassapaug," and the eastern boundary of the town is said to be "four score rod eastward of the easternmost of the pond." ...and its name is mentioned more frequently, perhaps, than any other of the aboriginal names belonging to the region. It is drained by the Quassapaug river, or Eight Mile brook, which empties into the Housatonic at Punkups. Mr. William Cothren, in his "History of Woodbury," speaking of Captain John Miner, says: "To the lovely lake on the eastern borders he applied the name Quassapaug, or 'The Beautiful Clear Water." This pleasant sheet of water, so cosily nestling among the verdant hills, furnished one of the first fishing places to the new settlers, cut off as they were from the seaboard by the boundless forests lying between them and the sea."
On a subsequent page, Mr. Cothren suggests another interpretation of the name— "Rocky pond"* — on the supposition that the first two syllables represent qussuk, meaning "rock" or "stone..."
In regard to the meaning of paug there can be no doubt. It denotes "water place" (pe-auke), is used for "water at rest," or "standing" as distinguished from "flowing" water, and is a frequent component of names of small lakes and ponds throughout New England.
But the proper interpretation of the first part of the word is somewhat uncertain. The Rev. Azel Backus, in 1812, in his "Account of Bethlem," interpreted the name as signifying " Little pond," apparently deriving it from okosse-paug; but in Dr. Trumbull's judgment "he certainly was wrong;" for "Quassapaug is not a small, but the largest pond in that region."
The author of this chapter, in his list of place-names in the Rev. Samuel Orcutt's "History of Derby," suggested that the name might possibly represent quunnosu-paug, that is, "Pickerel pond," and found incidental support for this opinion in Mr. Cothren's reference to the good fishing which the lake furnished to the early settlers. Dr. Trumbull, in his "Indian Names of Places in Connecticut," rejects this interpretation (but on insufficient grounds) and proposes another. He says it " may have been denominated k'chcpaug, that is, 'greatest pond'—a name easily corrupted to Quassapaug." Such a change does not seem an "easy" one, but there is documentary evidence in support of this interpretation. In a report concerning boundaries, made by the agents of Woodbury and Mattatuck, June 29, 1680, we find the expression, "the great pond, commonly called or known by the name of Quassapaug." It would seem as if here the Indian name and the English translation of it had been brought together.*
Quassaconkanuck Pond, Washington County, R. I. Eastern Niantic, "stone fence boundary mark," or "turning place at the stone wall."
Quassapaug Pond, New Haven County, Conn. Quinnipiac, "stones in the pond," or "gravelly pond." But if an abbreviation of Kehtequasset, "the largest."
“Quassy was not always the amusement park it is today. Native Americans once populated this large area. The Native Americans translated Lake Quassapaug to mean "Big Pond" or "Big Rock". In colonial days King George (1690-1770) granted one Abel Wheeler control of the lake. During this time a fisherman could rent a boat and leave his horse in a stall with plenty of hay for as little as 50¢ a day. Fisherman enjoyed the lake because they claimed that they caught fish by the buckets full...”
“Lake Quassapaug proved popular with indigenous populations and, later, with European settlers for the abundance of fish it provided. By the end of the 19th century, entrepreneurs began erecting businesses around the lake to take advantage of this draw while the well-to-do built summer estates on nearby slopes. In 1901, a writer for Forest and Stream; A Journal of Outdoor Life pronounced, “Lake Quassapaug is the handsomest bit of natural water on the earth,” but the author also foresaw changes to the area. He lamented: “One feature of the lake that is likely to be destroyed by a projected line of electric cars from Waterbury. Its sequestration has been a taking feature in the estimation of those who enjoy absolute rest and quietude. It is feared the trolley will introduce an objectionable element, but the wheels of progress cannot be stayed….” https://connecticuthistory.org/quassy-one-of-the-last-of-the-old-time-trolley-parks/