Prior to synthetic fabrics and built-in water-resistant properties people were tasked with finding ways of making clothing water resistant. A number of different natural methods were used: from beeswax to soap, or tars and various greases. Over time the waxing of clothing has become less a necessity and moreso an aesthetic.
Here's a simple tutorial on how to wax a beat-up shirt to give it new life. Or a way to get extra prepared for that next trip into the wilderness.