In essence, a huntard is a Hunter who does not know how to play his class well. While slang to describe badly-played classes is common, hunters have this stigma above all others.
Various reasons seem to exist to explain this. The class has an intuitive battle strategy, easily dumps aggro (more than likely onto the healer when in a group) and allows flexibility with weapons while also fighting long range. Huntards are especially disliked in low-level PUGs, where mismanaged growl-enabled pets can cause annoying and expensive wipe (versus the pet-using, squishier, and presumably more careful Warlock). Hunters also excel at solo PvE, compared to more group-oriented players like healers or tanks, and don't necessarily have grouping experience even at higher levels.
Huntards are also infamous for rolling Need on items and equipment not considered true upgrades, if not outright ill-suited to their class, usually under the excuse of being used as "statsticks". One example of this is a hunter rolling need a 1-handed melee dps weapon and claiming, "It will help me get out of tight places." In addition, huntards are prone to frequent melee combat, something normally only done in emergencies. This not only puts their safety at risk and diminishes their damage output, but can lead to unnecessary competition in group loot rolls with other classes like rogues and shamans. They are notorious for being self-centered and egomaniacal, never taking the blame for a wipe. They have tricked themselves into believing that they are the core of the group, but unfortunately often control its failure.