To some this may mean nothing, but with the family I grew up in it had a lot of meaning behind it. It's simply hand on forehead with fingers spread. This would be the sign given after one of us would, well you know, fart and we would hold it on our heads until everyone in the family followed with the same sign on there head. The last one to do it "ate it." So a typical scenario would be at dinner, and one of us would silently put our hand up, everyone would catch on, and when the last person did it we would all yell, YOU ATE IT. You never wanted to be this person. It's the same concept as not wanting to be the wrotten egg at the end of a race, you aren't sure exactly what it means to be the a wrotten egg but you know you don't want to be. (front View)
(side view)