He claimed he could run 100 kilometers in a day, which was a complete whopper.

whopper 是名词，来自动词 whop（重击、打），加上后缀 -er 形成名词，原指"巨大的东西"。在俚语中引申为"弥天大谎"或"夸张的谎言"，强调谎言的程度非常离谱。