Comes from the following situation: The boss asks you to bend over the desk so he can fuck you in the ass. You aren't quite bent over enough though, so he asks you to get something out of the drawer on the other side of the desk. Even further, and you are getting it from the "bottom drawer", thus, with your ass fully exposed and vulnerable to your employer.
In extreme cases, you may be asked to get something from the "Z folder" in the bottom drawer, which, being at the back of the drawer would put you in the most compromising position of all.
I got so Bottom Drawered today!