The Higher Noon
Point your crosshair at a medic to mark them for death. Also, you'll look like a f2p which is fun.
The Higher Noon is a custom secondary weapon for Engineer. The weapon is a selective-pointing Wrangler. When the crosshair is aimed at a Medic with The Higher Noon deployed, the Medic will be marked for death as long as they remain pointing at the Medic with The Higher Noon, going away if the weapon is holstered or the Engineer looks away. The weapon is commonly used for teamplay, with one Engineer marking Medics and another firing at them.
The Higher Noon was originally a pistol that fired shots that marked Medics for death for 3 seconds with lower damage and clip. It was changed into its current design because its intended teamplay use was not being accomplished and instead players simply marked for death then switched to another weapon. This is still possible to do by shooting a projectile then deploying the Wrangler, but is much more difficult than previously.