The word "但是" is formed through the morphological combination of two distinct, existing characters: "但" (dàn), which historically carries a sense of "only" or "merely", and "是" (shì), which means "to be" or "correct". They are fused together as a fixed, disyllabic compound where the original individual meanings semantically blend and shift to create a new, unified meaning of contrast or exception, effectively functioning as a single lexical unit meaning "but" or "however".