A meanderthal (as a close cousin of the passhole) is a driver whose rate of speed is slower than the actual rate (not necessarily the legal speed) of the traffic for the lane they are in (e.g. in an HOV lane that is going slower than non-HOV traffic to the right). The driver may even change lanes without additional speed, nor showing intention to pass another vehicle (e.g. in the middle lane of three lane highway going slower than traffic in that lane, and the right adjacent one). The driver is either using a cell phone, is from a stereotypical "nervous" driving group, or (worst case) intentionally being a jerk.
This lack of forethought, awareness, and consideration for other drivers causes disruption, and occasional road rage, creating a danger to themselves and other drivers.
"Another friggin' meanderthal is LOST in the HOV lane again!"
Kelly: I know, he’s being a total meanderthal.