When someone is doing or wearing something and dispalying much attitude, thinking they are the ultimate, but the result is actually the oipposite. The inference is that just like you can't see air, the person can't see that they areen't really fierce at all. The exact opposite of "fierce" Used mostly by gay men in the 80's and 90's.
"Did you see Miss Thing marching around in those pumps? She is so air-firece."