你很愚蠢的英文表达是 "You are foolish." 来源可能源自于拉丁语中的 "folium" 或 "foolus"，意为 "草"或 "傻子"。

例句：

You are foolish to believe that the Earth is flat. 你相信地球是平的，真是太愚蠢了。

She was so foolish to quit her well-paying job without a backup plan. 她没有备用计划就辞掉高薪工作，真是太愚蠢了。