This word comes from the Scottish “flendris,” but it can be traced to the Norwegian “flindra,” meaning “chip, splinter,” or the Dutch “flender,” meaning “fragment.” It’s a plural noun, because there’s no usage for “flinder,” meaning a single fragment. If you need a singular form, try one of the synonyms “splinter,” “scrap,” “morsel,” “snippet,” or “sliver.” But don’t use “smithereens,” as there’s no usage for a singular version of this slang word either. No one ever smashed something to a single smithereen or flinder.