Someone who drives poorly and you hope "someday" they will get driving lessons and finally learn to drive.
A replacement for Sunday driver directed at people who are driving poorly, very slowly in the left lane or just dithering and dawdling, not signaling or just plain unaware of their surroundings and what is the appropriate response. Often holding a cell phone! A crash waiting to occur!
Look at that someday driver, maybe he'll finally learn how to use his signal.
Let's pass that someday driver before he loses control and drives some poor innocent off the road!