A co-worker was just telling me this story in which his normally very straight-laced wife blurted out to her teenage son, in a moment of frustration, "I'm not just blowing smoke up your ass!" Well, this boy had never heard this expression before and was dually mortified; that his sweet mother had said such a thing and really? Just what the hell did that mean, anyway?
So my co-worker did what any other red-blooded person living in the 21st century would do: he googled it. Here is what he came up with, courtesy of Urban Dictionary:
Most literally translating to "trying to get a reaction" out of somebody. Originated from a 16th century technique to determine death by literally blowing tobacco smoke into someone's anus via a long pipe. A reaction indicated that the person was still alive.