A person who frequently attacks others in speech or writing, but who poses no intellectual threat whatsoever. The motivation of this type of person can usually be accurately construed as a desire to be obnoxious and offensive.
Origin: The phrase "porch dog" is used to refer to dogs that sit on front porches and bark (vigorously and fruitlessly) at passersby, but who pose no physical threat.
Yeah, that guy has a scathing response to just about everyone who posts in this forum. He's a real porch dog.