Is legs a common or proper noun?

The noun 'leg' is a common noun, a Word for any leg of any kind.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Jack Moran, aka 'Legs Diamond'
• Leg Lake, Hiawatha National Forest, MI
• Leg Horn Road, Kerhonkson, NY
• Leg O' Mutton Inn, Yelverton, UK
• 'Legs' by William Kennedy (the story of Legs Diamond)