Question of the hour

For reasons as yet unclear to me, I've had the phrase "blowing smoke up my ass" stuck in my very tired brain all morning. Specifically, I'm wondering how on earth that phrase came to be. I haven't Googled it yet because I'm having entirely too much fun trying to guess its origins on my own. Does it relate to some medieval torture technique? A primitive medical procedure? An ancient seers' ritual? Anyone else have a guess?