It began in London with Porter, a beer formed from the many varied dark ales of the day and transformed into the world’s first industrial-scale style, once drunk with the same ubiquity as pale lagers are drunk today. It has a history just as interesting as IPA, perhaps even more so (did you know more porter sailed to India than IPA?), and at different times it’s been different things, though at it’s peak it was a robust, strong, dark beer aged for many months in massive vats.
Stout started as an adjective meaning ‘strong’ and a couple of centuries ago it was used in the same way we might currently use ‘session’ or ‘imperial’, where, for example, stout porter was a stronger version of a regular porter. Over time it came to mean a strong dark beer, as it travelled and took on different qualities. It also picked up a reputation as a restorative, wholesome drink, so when World Wars implemented ingredient rationing, the now old-fashioned porters were forgotten for the healthful stouts, causing porter to all-but disappear for two generations, before small breweries brought them back.