Having “guts” or “pluck” means having courage or backbone, while having neither means lily-livered cowardice, and they are all references to intestinal fortitude.
Guts, of course, are internal organs while pluck is collectively the heart, liver, and lungs.
Lily-livered comes from the belief that fear drains blood from the liver, making it white.
It was once believed that these internal organs, specifically the heart, were the source of a person’s character.
In the eighteenth century, the pluck contained the heart, liver, lights, melt, and skirt, lights were lungs, melt was the collected blood, and the skirt was the diaphragm.