A staple Old Folk agressive move. Shaking the fist lets people know they are disgruntled but haven't the means to kick your ass. Directly related to the wave off, the fist shake is a time honored right for senior citizens, and adored by the young.
When I burned out in front of Old Man Stephens house, he gave me such a fist shaking that I thought he was gonna bust a jugular.
What old people do from the porch when little kid's baseballs break their windows.
Those little rat bastards broke my kitchen window again, so I gave'em the old fist shake from the porch to scare them off.