"不聪明" 的英文等价词是 "stupid" 或 "unintelpgent"。
"Stupid" 来源于古英语 "stupian"，意思是"智力低下"。"Unintelpgent" 是由 "un-"（否定前缀）和 "intelpgent"（聪明的）组成的词。
例句:
He was so stupid that he couldn't even spell his own name. (他太蠢了，连自己的名字都不会拼写。)
The plan was unintelpgent and doomed to fail. (这个计划不聪明，注定失败。)