The term 'SLIPPERY' as an eel refers to the physical characteristics of eels, which are known for their slimy and SLIPPERY skin. Eels have a unique body shape that allows them to move quickly and easily through water, making them difficult to catch. They also have a mucus-like coating on their skin that helps them to avoid predators and stay hydrated. This coating also gives them a SLIPPERY texture, which is why they are often referred to as 'slipper eels.'