To explain this one is a little tricky. You will see various spellings in both Japanese and English!
シークヮーサー or シークァーサー or シークワーサー all mean “Citrus depressa,” a type of lime grown here in the Ryukyu islands. It is romanized as shikwasa (most common), shikuwasa, or shiquasa. In Japanese, it is also known as ヒラミレモン hirami lemon.
The name comes from 2 parts: 「シー」shii which means sour (酸い suppai in Japanese) and 「クワス」kuwasu meaning eating/to eat (食わし kuwashi).
It is small and similar to a lime: a sour, citrusy flavor. When I first saw them I thought maybe they were calamansi (which I adore), but the taste is a little different than that. Since the taste is so sour, most people add sugar to the juice to balance it. I prefer less sugar since I really like sour flavors.
You can buy shiqwasa in grocery stores and farmers markets (when they are in season*, or the 100% juice year-round. There is a shiqwasa “park” (tourist attraction) up north in Nago where you can try fresh juice.
*Shikwasa are generally in season from August through February, though they are most sour early in the season. Towards January and February they get less sour and can be eaten just as is.
You can buy just bottled PET bottle drinks of shikwasa drink which is diluted with water and tons of sugar added. It is a popular flavor for various sweet desserts/snacks and ice cream. There is even Okinawa limited potato chips with shikwasa flavor (kind of like salt and vinegar but a citrus-y acid!). Oh, and even Orion makes a shikwasa beer… it is not that great, but it is basically Orion beer with a tiny bit of shikwasa juice (and I think part of the citrus peel) added.