The word 夹心 (jiāxīn) is a compound noun formed by combining the verb 夹 (jiā), meaning "to clamp" or "to sandwich between two things," with the noun 心 (xīn), meaning "heart" or "center." Literally, it describes the action and result of one substance being tightly enclosed in the middle of another, much like a filling "sandwiched" between two layers. This morphological process creates a vivid conceptual metaphor, which is most commonly applied to food items like 夹心饼干 (sandwich biscuits) or 夹心糖 (filled candies), and can be extended metaphorically to describe any layered or intermediate state, such as 夹心阶层 (the "sandwich class" referring to middle-income families).