A coward is one who turns his tail and runs: literally!
The English coward comes from the old French coart. Coart, in turn, comes from coe, meaning “tail” (from the Latin, coda for the same), plus the -art suffix just refers to a person doing that (think, braggart). A coward shows you his tail and turns the other way!
Interestingly, from the Latin coda, we also get the Spanish for tail, cola. And from the French coart, we get the Spanish word for coward, cobarde.