The name chickenpox has been around for centuries, and there are a number of theories as to how it got its name. One is that it's from the blisters that are seen with the illness. These red spots — which are about 1/5 inch to 2/5 inch (5mm to 10mm) wide — were once thought to look like chickpeas (garbanzo beans).
Another theory is that the rash of chickenpox looks like the peck marks caused by a chicken. But, in case you were wondering, chickenpox can't be caught from a chicken!