“story”有“编造的故事”之意，a tall story 就可以理解为以夸张的方式编造出的长篇故事，令人难以置信之事。

【例句】
I'm still shocked to hear the tall story that John is leaving his wealthy girfriend.He is such a peacock.
我依旧对约翰跟他有钱的女友分手的事感到难以置信。他是如此爱慕虚荣的人。

What you said just now sounds like a tall story.
你刚刚讲的听起来像个荒诞的故事。