You are a chicken意思是：你是个胆小鬼，你是个懦夫。

【例句】
You are such a chicken. If you don't agree with our manager, why don't you tell it to his face?
你真是个胆小鬼。如果你不同意经理的观点，为什么不当面告诉他?

You are a chicken. There is no need to be afraid of a little dog.
你是个胆小鬼，完全没必要害怕一条小狗嘛。