'Blowing Smoke Up Your Ass': The Literal Meaning/Medical Practice Behind The Idiom

Royal Humane Society resuscitation apparatus. (Science Museum, London)

To "blow smoke" has had a few definitions over the years, and the end of the phrase (as well as its meaning) varies. You can "blow smoke in someone's face" to obfuscate a situation. You can "blow smoke" to lie or exaggerate the truth. Yet "blowing smoke up someone's ass" has a very specific meaning: to praise someone in a way that is not sincere. The first two meanings draw a clear reference to how smoke makes it difficult to see, but it's unclear at first blush how butts got involved. The answer, as it often does, lies in history.