chicken–I’ve talked about “chick” used to refer to people, including oneself. There is another meaning, however, that we are more closely associated with: coward. Interestingly, chicken wasn’t used that way in the 19th century. The first reference in terms of cowardly is 1933. I would not have thought that. Nor did one “chicken out” until 1934.
However, one could be 1) chicken-livered, which did mean cowardly. The first reference on this was 1871, by Mark Twain. There weren’t a lot of other references, however one could also be a 2) chicken-gizzard–a coward. There’s only one reference to this, in 1851.
The only other reference I could find that came even close to cowardly was “gone chicken” or “dead chicken” (1863) but that seems to be mostly about someone being in a dismal, probably unrecoverable, situation, often in reference to oneself. “Once Harold found out that she’d been unfaithful, she was a dead chicken.”
So. . . I could very well have incorrectly used chicken in one of my books. Too late now, but I’ll be watching for this in the future!