The expression "showstopper" originates in the world of theatre, particularly musicals, where it indicated a good thing.

In this context, a showstopper was an outstanding performance, such as a song or dance piece, that was so well received by the audience that the show had to pause until the applause ended. The show was literally stopped. The first example that comes to mind is the anthemic "You'll Never Walk Alone", from the musical "Carousel".

This is one of the few examples of an old expression that continues to be used, but whose significance has changed completely.