From Longman Dictionary of Contemporary Englishthat’s a good onethat’s a good oneused to tell someone that you do not believe something they have said and think it is a joke or a trick You won $50,000? That’s a good one! → good
Examples from the Corpus
that’s a good one• My car's on fire? Boy, that's a good one!• The idea of John getting sick, the idea of John coming down with something: that's a good one. 