Today I learned

That unusual therapy is all very well—or perhaps not—but its existence among some pre-Columbian peoples of the Western Hemisphere and some post-Columbian peoples of eighteenth-century Europe does little to explain why the slang expression "blow smoke up [someone's] ass" emerged only in the 1940s or later. My guess is that the historical practice is only coincidentally related to the modern expression.

But if the weird medical practice isn't the source of the slang term, what is? I suspect that the answer is amplification: sometimes a slang term that has been around for a while acquires new cachet thanks to a snappy word replacement or an edgier extension. But if this simply a case of amplification, a couple of other phenomena are likely: (1) we might see alternative extended versions of "blow smoke"—especially in the period before "blow smoke up [someone's] ass" comes into frequent use—that use less startling words than "ass"; and (2) we ought to see "blow smoke up [someone's] ass" being used not just for "blowing smoke" definition 2 above, but for definitions 1 and 3 as well.