"铁头" (tiě tóu) 不是 "内鬼" 的英文表达是 "Iron Head is not an insider."
这句话的来源是中文俗语，意思是某人没有受到内部的操纵或指使。
例句：
Iron Head is not an insider. He has no idea what's going on behind the scenes. (铁头不是内鬼。他根本不知道幕后发生了什么。)
I can assure you that Iron Head is not an insider. He's just a bystander. (我可以向你保证，铁头不是内鬼。他只是旁观者。)