The phrase always makes me grin—it's such a delightfully odd farewell that somehow feels sincere. In Douglas Adams' universe, dolphins are the clever, semi-mysterious inhabitants who know more than humans give them credit for. Before Earth gets demolished (one of many sardonic plot points in 'The Hitchhiker's Guide to the Galaxy' saga), the dolphins leave, and their parting message is literally 'So long and thanks for all the fish.' It's a perfect little blend of wry humor and affectionate goodbye, and Adams later used that exact line as the title for his book 'So Long, and Thanks for All the Fish'.
Beyond the books, the phrase took on a life of its own. I see it slapped on T-shirts, whispered as farewells on forums, and tucked into emails when someone leaves a job or a project. It carries this playful gratitude—thanking someone for the good bits while acknowledging that things are ending. For me, it's the kind of goodbye that doesn't scream melodrama; instead, it winks and nudges, reminding you to appreciate the absurdities. I still chuckle if I spot it in a movie end-credits or a band's setlist, and sometimes I throw it into a goodbye card just to watch people smile.