Slang for a type of raucous laughter in which the balls jiggle uncontrollably. Can be experienced by a woman if her breasts (i.e. female balls) also jiggle uncontrollably.
When an event is so ridiculously funny that the level of its humor transcends the communicative abilities of the English language. When "funny," "haha," "roflcopter" and even "lollersk8r" do not suffice. Then, it is time for the hilarbear to roar.