That Dog Won't Hunt - Southernism
Contrary to what you may think, this Southernism has nothing to do with old or ornery beagles. Rather, it is a Southerners way of saying, "That plan won't work," or "That excuse won't fly."
So, the next time someone in your circle of friends comes up with what they deem is a bullet proof plan but makes no sense to you, feel free to let him/her know, "That don't won't hunt." Or when you're being fed a lame excuse, simply reply, "That dog won't hunt." If you're not a resident of the South, stand back and watch the quizzical reaction.