Literally: truck, or vehicle.
Remember the word for lighting/electricity (atsiniltłish)? If you can imagine the tłish as the sound of a loud crash, then imagine chid as the sound of an engine chugging.
That’s precisely how trucks, and engine-operated vehicles, are commonly described. Back in the day, the sound that loud engines produced became their defining characteristic for Navajo speakers. So, they simply adopted that characteristic to form the word itself.
You may also hear chidii in some places, or in a more relaxed setting.