If you watch old westerns, the doors on a saloon (the bar) swing outward from the center as someone walked through them. The name was originally given to cars

If you watch old westerns, the doors on a saloon (the bar) swing outward from the center as someone walked through them. The name was originally given to cars that have normal opening front doors and suicide rear doors which would have a similar appearance when you opened them. Eventually it started being applied to any 4 door regardless of how the doors opened.