The word 应当 is formed through the morphological process of compounding, where two independent and semantically similar characters, 应 (yīng, meaning "should" or "respond to") and 当 (dāng, meaning "ought" or "be fitting"), are combined. This pairing creates a single, cohesive unit where the meanings reinforce each other, resulting in a strengthened and somewhat more formal expression of obligation or appropriateness ("should" or "ought to") than either character might convey alone.