Middle English squirel, from Anglo-French escurel, esquirel, from Vulgar Latin *scuriolus, diminutive of scurius, alteration of Latin *sciurus, from Greek skiouros, probably from skia shadow + oura tail — more at shine, ass
: a small gnawing animal that is a rodent usually with a bushy tail and soft fur and strong hind legs used especially for leaping among tree branches
Word History of SQUIRREL
When a squirrel sits up, its long tail curves up and over its head and sometimes casts a shadow. The English word squirrel comes ultimately from the Greek word for a squirrel, skiouros, which is thought to mean “shadow-tailed” (skia, “shadow,” plus oura, “tail”).