“Swiftboating is not going to work this time. And the reason it’s not is, No. 1, I’m going to smack ‘em right square in the chops.” — Joseph Biden, "Biden: Hillary Clinton ‘might have been a better pick’ for VP"
"Swiftboating" is what Ayn Rand called an "anti-concept." Everyone on the left uses it indiscriminately but it's never really defined. People seem to use it to mean "character assassination" or "smear campaign" but it is often trotted out to stifle negative, though truthful, political discussions.
Dismissing an opponent's charges or attacks as "Swiftboating" is a sure way to squelch the attacker as well as plant the seeds of doubt in the listener without refuting or countering the attack. Whenever you hear the term bandied about, it's time to critically examine the attack rather than dismiss it out of hand. Also, be aware that the one invoking it may be trying to pull something over on you as well.
(Side note: is anyone else troubled by a Senator and VP candidate threatening violence on anyone attacking Obama? Maybe he's channeling Preston Brooks.)
[UPDATE: Here's a good examination of the term and its proper usage.]