An elongated blunt object used to beat the living crap out of someone/something. The more unusual the object is, the more worthy of the term. Ceremonial whompi (plural form) are used specifically toward the destruction of anything festive in nature, such as Christmas Lawn ornaments.
Holy Shit!! Did you just see that lunatic hit that mall Santa over the head with that gigantic whompus? It looked like the knobby end of a hat rack painted black.