get out of town = you are mistaken, do not expect us to believe you
You saw a Martian? Get outa town! I don't believe it.https://en.wiktionary.org/wiki/get_out_of_town
get out of town
Expression of incredulity. http://english.stackexchange.com/questions/30...
Here the phrase is used in a nice explanation of the term "Shut up!" to express amazement:
"The Wall Street Journal reported about this use on May 1, 2003 in 'Amused? Want to Hear More? One Term Says It All: "Shut Up!" reported on its mainstreaming and sheds some more light on its origin. Some extracts:
"Not too many years ago, the unrude use of 'Shut up!' might have baffled linguists and just about everybody else. But the term has now made its way from schoolgirl chatter to adult repartee and into movies and advertising. People use it as much to express disbelief, shock and joy as to demand silence. In some circles, it has become the preferred way to say 'Oh my God!' 'Get out of town!' and 'No way!' all at once."