The word "于是" is formed from the character "于", which originally functions as a preposition meaning "at" or "in", and "是", which serves as a demonstrative meaning "this". In classical Chinese, the combination "于是" was used literally as a prepositional phrase meaning "at this" or "in this situation", often referring to a specific context or moment. Over time, this phrase grammaticalized into a fixed conjunction, where "于" sets the circumstantial frame and "是" anaphorically refers to the prior event, thus deriving the consequential meaning "so" or "therefore".