Xystus \ZIST-us\ , noun;
1. Ancient Greek and Roman architecture: A covered portico, as a promenade
2. Ancient Rome: A garden walk planted with trees in a villa
Today I noticed that I have at least one entry for each letter of the alphabet except 'x'. Consider that error fixed!
This word is first attested in 1664 from Latin xystus, which was borrowed from Greek xyotos ("scraped, polished") from xyein ("to scrape polish"), which ultimately derives from Proto-Indo-Europena *kes- ("to scrape").
Side note: Lots of 'x' words are Greek