Expression: I’d give my right arm for…

There is a popular expression “I’d give my right arm for…” meaning “I’d really love to have…”. It means that it (whatever it is) is so important to me that I would gladly lose something as important as my right arm in exchange for it. For example, “I’d give my right arm to live in a warm climate,” or “I’d give my right arm to be a good singer.”
There is an ironic joke version of this: “I’d give my right arm to be a good guitarist.” Do you understand why that is ironic?