1. The caption of a popular "loldog" (dog version of lolcat) showing a dog attempting to drink from a lawn sprinkler. Word refers to the sound the dog presumably is making as it angrily (and pointlessly) attempts to subdue the water stream.
2. A one-word stand-in for any angry, incoherent, pointless reaction, comment, or attack. In online forums, the loldog referenced in (1) is often used to make the point that someone's position is incoherent, illogical, and or pointless. Simply writing 'WHARRGARBL' is also sufficient to make the point, and is especially effective if the target of your insult is one of the few people unfamiliar with the loldog. Note that incoherence or pointlessness is required for a proper WHARRGARBL reference. An angry, but otherwise logical, argument is not a WHARRGARBL. An argument that takes a reasonable (or at least arguable) position, but does so with an incoherent rant IS a valid WHARRGARBL. Coherence and logic (or the lack thereof) is in the eye of the beholder, of course. On the internets, any argument that disagrees with yours may be a valid WHARRGARBL.