If you describe something as implausible, you believe that it is unlikely to be true.
例句
I had to admit it sounded like an implausible excuse...我得承认那听起来像是个难以叫人相信的借口。It seems implausible that the projects would have gone ahead without her backing.这些项目没有得到她的支持就开始进行了,这似乎不合情理。