There's a back-and-forth email correspondence between Sam Harris and Noam Chomsky discussing ethics here. In one of the emails Sam says to Noam:
I trust that certain of your acolytes would love to see the master in high dudgeon—believing, as you seem to, that you are in the process of mopping the floor with me—but the truth is that your emotions are getting the better of you. I’d rather you not look like the dog who caught the car.
I've found only one site online that gives a meaning of it:
n. A person who has reached their goal but doesn’t know what to do next.
This idiom is based on the strange habit that some dogs have of chasing cars that are passing by on a nearby road. What on earth would one of these crazed canines do if it actually caught a car? This idiom is also seen as the dog that caught the truck (1993) and the dog that caught the bus (1994).
However this meaning doesn't make sense to me in the context. Sam is accusing Noam of being belligerent and cantankerous, and I don't see this as Sam telling Noam that he'd hate to see him be "the one who has achieved his goal and now doesn't know what to do."
I also found some user definitions on Yahoo Answers, but the meanings vary. The top answer there is:
the impossible has been accomplished.
The second answer is:
It can be taken two ways .
One.......it means you finally did something, you have tried many times and failed...........
or it can be a euphemism, for.........the Dog is dead.
Because the dog actually CATCHING the car.....usually doesn't end well for the dog.
Achieving the impossible doesn't seem to fit in the Harris-Chomsky discussion.
However, the dog being dead makes a bit more sense, as Sam is telling Noam that his manifest aggressiveness is not making him look good in front of their readers, and so the dog being dead could be figurative for Noam's attacks backfiring on him and making him look bad.
I'm having trouble knowing what is meant by this in the email exchange, and also in general. Stephen Colbert seems to use it in the way Wordspy defined it:
Stephen Colbert likens Trump team to the dog that caught the car
Trump and his team, he continued, are “like a dog who spent his whole life chasing a car — now he has to drive the car.”
Entertainment Weekly article
Does this most generally mean what Wordspy says? Also, is my reading of Sam Harris' use of it to mean "dead dog" right?