Per Dictionary of Word Origins, Jordan Almond:

"Lallapaloosa" comes from a provincialism of County Mayo, Ireland -- allay-foozee, meaning a "sturdy fellow." The Irish adopted it from the French -- who, when they landed at Killala in 1798, repeatedly shouted, Allez-fusil! -- meaning, "Forward the muskets."

[got-a-new-etymology-book emoticon]