Admit it. You've always wondered why a trailer with two or four wheels of its own is called a "fifth wheel", right? Still, we know what it means. It's an RV travel trailer that is towed from the bed of a pickup truck or a truck tractor, allowing for more driving stability and interior space. But there was a time when the term "fifth wheel" was relatively unheard of and much more literal.
Check this out. And then you might be wondering why what seems like a great idea didn't go into mass production.