Sprauncy (pronounced "shprauncy") is one of those unusual Jewish words that appears to be a genuinely original invention of British Jews. American Jews have never heard of it. Neither have most English non-Jews.
But I have encountered a wide cross-section of British Jewry from university students to my grandparents who could use sprauncy competently in a sentence, such as: "Just look at her, so pleased with herself in that sprauncy pink outfit"; or "The black tie ball is the spraunciest event in the J-Soc calendar", or "What a sprauncy barmitzvah that was; even the waiters and waitresses were dressed up like characters from Harry Potter".
In other words, sprauncy connotes something between stylish and opulent.
The word sprauncy does not appear in any dictionary that I know if, and so its origins are obscure. Here is my best guess. The rabbinic word shapar means, literally, polished, well-rounded or finished. From there it came to mean beautiful or attractive. The Talmud frequently uses the expression shapir amar Rav X to say that "Rabbi x gave a fine or convincing answer or explanation". My theory is that the word shapar, meaning beautiful, was crossed with the English "fancy" to create this hybrid that describes something pretty, but ostentatious.
But this is sheer speculation. If anyone has a better idea I would be happy to hear it.