In this sense, chinless does not mean a person has no chin. In British English, especially older, jokey, or newspaper English, it means they have no backbone: they will not stand up for anything, and they fold as soon as life gets hard. The picture behind it is an old class joke. People used to mock posh young men with weak, receding chins as soft, spoiled, and useless when something needed doing. The set phrase is a chinless wonder. So the word is not only “scared”; it is ineffectual, a bit wet, and it often carries a class sting. You will hear it far more in Britain than in the United States. It is informal, a little old-fashioned, and unkind if you aim it at a real person. Though actually, a few people still use chinless almost literally for a very small chin — that is a different, physical reading, not this one.

沒骨氣的（英式口語，常帶嘲諷；不是在說下巴）