A now stereotypical, thereby ad-hominem label for an individual who is sceptical about world events. Such scepticism largely emanated from the events of 9/11.
Some individuals embrace the term while others see it as a convenient label to paint all with the same brush.
Pro: I'm a truther, I look at the empirical evidence that wasn't in the official 9/11 commission report.
Con: You're just another one of those "truthers"! Inside job people.