early 14c., from Anglo-French esquirel, Old French escurueil "squirrel; squirrel fur" (Modern French écureuil), from Vulgar Latin *scuriolus, diminutive of *scurius "squirrel," variant of Latin sciurus, from Greek skiouros "a squirrel," literally "shadow-tailed," from skia "shadow" (see Ascians) + oura "tail," from PIE root *ors- "buttocks, backside" (see arse). Perhaps the original notion is "that which makes a shade with its tail," but Beekes writes that this "looks like a folk etymology rather than a serious explanation." The Old English word was acweorna, which survived into Middle English as aquerne.
"small striped squirrel of eastern North America," 1829 (also chitmunk, 1832), from Algonquian, probably Ojibwa ajidamoo (in the Ottawa dialect ajidamoonh) "red squirrel," literally "head first," or "one who descends trees headlong" (containing ajid- "upside down"), probably influenced by English chip and mink. Other early names for it included ground squirrel and striped squirrel.
popular name of the American red squirrel, 1829, echoic of its cry.
c. 1600, from German Hamster, from Middle High German hamastra "hamster," probably from Old Church Slavonic chomestoru "hamster" (the animal is native to southeastern Europe), which is perhaps a blend of Russian chomiak "hamster," and Lithuanian staras "ground squirrel." The older English name for it was German rat.
1835, jocular formation, perhaps based on fantasy.
There is an indescribable complaint, which will never allow a moment's repose to mind or body; which nothing will satisfy—which allows of no beginning, and no ending—which wheels round the mind like the squirrel in its cage, ever moving, but still making no progress. It is called the Fantods. From the diagnostics, we pronounce Lord Brougham incurably diseased with the Fantods. [The Metropolitan, London, October 1835]