The word "一共" is formed by combining two characters whose core meanings create a new, unified concept. The first character, "一" (yī), fundamentally means "one" or "a single whole". The second, "共" (gòng), carries the essential idea of "together", "shared", or "in total". When placed side-by-side, their literal sense merges to express the notion of "one whole together" or "all combined into one", which naturally gives rise to its functional meaning of indicating a sum total or an overall aggregate of items.