A gastropub is a pub that serves very good food — not only beer and a packet of crisps, but proper, often quite ambitious cooking. The word was born in Britain in the early 1990s, when some pubs started serving restaurant-quality meals. It is informal-to-neutral, and it still sounds most at home in British English, though other countries have borrowed it. The gastro- here is the “good food” gastro- from gastronomy, not the medical one. Though actually, as the idea spread, some places called themselves gastropubs while serving food that was only a bit nicer than average — the word got stretched.