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1.
What you get when you travel by car for long distances.
You for sure will have "car butt" after driving from NY to LA on vacation!
by ushka July 20, 2009
 
2.
When a passenger falls asleep as soon as the vehicle begins motion.
"The baby has car butt. Take him on a drive to put him asleep."
by Keybo4rd_Warri0r June 27, 2013