The Chainsaw is a scrapped Duke Nukem Forever weapon.
The Chainsaw appears to be a small chainsaw that covers the entire hand. It is obviously supposed to be a melee weapon.
No footage of it in action has ever been seen, making it impossible to see how powerful it would be.
The Chainsaw appears to have been completely scrapped, as no sign of it can be found in the 2011 footage nor the 2009 leaked materials.