means A small to medium-sized long-tailed rodent, larger than a mouse, often found near human settlements and frequently regarded as a pest.

from From Old English 'ræt,' with cousins across the Germanic languages — German 'Ratte,' Dutch 'rat' — and possibly relatives in the Romance world like French 'rat.' The deeper root is murky; the word may ultimately echo something gnawed or scraped, but no one can prove the trail back far. The sense of 'rat' as a traitor or informer is much younger, an 18th-century slang twist, riding on the animal's reputation for deserting sinking ships.