Why did cowboys put egg shells in coffee?

I'm curious, why on earth would cowboys put egg shells in their coffee? Was it a traditional practice passed down from generation to generation? Did it serve some practical purpose, like improving the taste or keeping the coffee hot longer? Or was it just a quirky habit of a bygone era? I'd love to know the reasoning behind this unusual custom, and if there's any truth to the rumor that it was actually a misunderstanding or a joke that got out of hand.