The word 'squirrel' has a playful and descriptive origin. It entered English in the early 14th century from the Anglo-French 'esquirel', which itself came from the Old French 'escurueil'. This traces back to the Vulgar Latin 'scuriolus', a diminutive of the Latin 'sciurus'. The Latin term was borrowed from Ancient Greek 'σκίουρος' (skíouros), a wonderfully literal compound meaning "shadow-tail", from 'σκιά' (skiá, "shadow") and 'οὐρά' (ourá, "tail"). This name was inspired by the squirrel's habit of sitting upright and using its large, bushy tail as a sunshade. Over centuries, the word's form simplified in French and English, obscuring its original Greek components while retaining its reference to the agile, tree-dwelling animal.

The word is a simplex loanword. Its form in English does not allow for segmentation into meaningful, reusable morphemes derived from its current spelling.