In the Oxford English Dictionary there are 19 separate entries for "butt" with various disparate senses, including a bundle of cloth, a headland, and a funnel-shaped wicker basket. Some of those entries have complex, multiple etymologies, but the word "buttock" is said to be derived from "butt, n.6" for which the etymology is given as: "Of uncertain origin."

So there you are. Happy to be of no assistance whatsoever!