The Razorback is an unlockable item for the Sniper. It unlocks when you have obtained 17 of the Sniper achievements.
The Razorback replaces the SMG in your secondary item slot. It's not a weapon, but while equipped, it will prevent one backstab attempt by an enemy Spy. The enemy Spy will be stunned for two seconds, and they will not be able to cloak or change weapons during that time. The Razorback is a one time use item - it does not regenerate like other items - and you can only get it back by respawning or returning to your resupply locker in your base.