Why anyone would endure such a treatment is an even greater puzzle. Do you mean it or are you just blowing smoke —comic strip "Sally Forth" 4 v phr by s To smoke marijuana or hashish: To blow smoke up someone's ass. The optional addition of "up [someone's] ass" to "blow smoke" in the definition 2 sense of the phrase but not to any of the others strongly suggests that the extra wording serves as an insulting intensifier rather than representing a phrase grounded in some sort of medical or physiological procedure. Blow smoke up someone's arse is used for both related meaning of lying and exaggerating insincere flattering:. Big as you are in the county.
Most of the discussions I've seen are in the context of the phrase's purported origins in a quasi-medical practice used either to revive drowning victims or to test for signs of life.
blow it out one's ass