Some assert that "buttload" was first used in print in the novel "Los Angeles Without a Map" by Richard Rayner in 1988, and that it's chiefly used in the United States. Fair enough. But what about its close yet more vulgar cousin, sh*tload? Merriam-Webster has that one going as far back as 1954, but they still blame Americans for it. The Brits had cleaned this one up by the 1990s, exchanging it for "shedload."
As long as we're looking up words, what about "boatload?" Could that have led to the "buttload" phenomenon? Maybe, but boatload has been in use since at least 1625, when it was used by explorer John Davis as recounted by Samual Purchas in "Purchas His Pilgrims." It really does just mean the number of people a boat can hold, and there don't seem to be any clear lines between boatload and buttload across more than 400 years.