Definition of but then
1 —used to introduce a statement that adds another and different thought to a preceding statement I'm surprised to hear that Tom has left the army. But then I suppose he never was the type to obey orders. —sometimes used to introduce an explanation for something His cakes are amazing—but then he did study with some of the best pastry chefs in the world.
2 —used to introduce a statement that tells about something different or surprising that happened next The team won the first two games, but then failed to win any of the next five.