The Spanish word 'ardilla' meaning 'squirrel' comes from Old Spanish 'arda' (also meaning 'squirrel') combined with the diminutive suffix '-illa'. The diminutive suffix '-illa' is commonly used in Spanish to indicate something small or cute, so 'ardilla' literally means 'little squirrel'.

While 'ardilla' itself is a relatively common word in Spanish, the diminutive suffix '-illa' appears in many simpler Spanish words. You might recognize it in words like 'tortilla' (little cake/torta), 'semilla' (seed, literally 'little seed'), or 'pastilla' (pill, literally 'little paste').

There aren't any common English words directly related to 'ardilla', as it comes from Old Spanish 'arda'. English 'squirrel' has a completely different etymology, coming from Ancient Greek via Latin and Old French.