Prior to changes in the 19th century, the mens shirt also served as their underwear. While there are sporadic other methods of protecting that area and if you go far enough back the shirt itself changes, we're more concerned with what we by modern standards would deem a shirt. Often made of linen, sometimes of wool, and eventually of cotton in the 19th century, the shirt protected the outer clothing from body residue, sweat, and odor. It came down to about the knees in length and was tucked between the legs before putting on breeches or trousers.

Things like smocks, hunting shirts, and working frocks are shirt-like but still worn over the shirt. Those are left untucked, but are all considered working outer garments and not underwear. The shirt doesn't lose its purpose of underwear until the later half of the 19th century when separate drawers become the popular alternative.