The word 单单 is formed through the process of reduplication, where the base character 单 (dān), meaning "single," "alone," or "only," is doubled. This repetition does not create a simple plural but instead functions as an intensifier or a focus marker. Grammatically, it shifts the word from an adjective into an adverb, emphasizing exclusivity or singling something out from a broader context. Therefore, 单单 means "merely," "only," or "solely," and is used to restrict the scope of a statement, highlighting that something applies to one specific case to the exclusion of all others.