The word 必须 is formed through the process of semantic compounding, where two characters with similar or reinforcing meanings are combined to create a single, stronger concept. The character 必 (bì) carries the core idea of "certainly" or "must", while 须 (xū) contributes the notion of "must" or "necessary". Their combination is not sequential but rather parallel, merging their individual connotations of inevitability and obligation into one intensified, non-negotiable sense of "must" or "have to".