The Braggin Banj' is a melee weapon especially for Engineers right at the front. This weapon is nothing else than the guitar from the "Meet The Engineer" Video.  It has a longer range than other melee weapons (like the eyelander), but it's also got a slower swing than other weapons. This weapon has the ability to create a tinnitus on the target which disables all sounds in the game. It disables sounds, character voices and even voicechat sounds for a few seconds. How long this tinnitus holds on depend on the range you hit the target. A Direct Contact with the Guitar creates a 5-7 second tinnitus, while a nearly missed hit creates one for 2-3 seconds. Maybe the HUD will be a little bit blurry on a obvious hit, but not strong.
The negative side of this weapon is, that you can't upgrade/speedup or even repair your sentries. Only friendly engineers can do that for you. And of course, the slower swing rate.
The taunt would be very obvious I think: The Engineers plays on his guitar a western or texas Song for a few seconds. Maybe there can be a easteregg with the L4D2 Banjo Sound or the Bonanza Soundtrack.