The word 对于 is formed through the classical Chinese process of lexicalization, where two independent grammatical and semantic units fuse into a single, inseparable compound. The character 对 originally carried meanings related to "facing", "responding to", or "correctness", while 于 functioned primarily as a preposition indicating direction, target, or relation ("to", "at", "in"). Over time, the frequent juxtaposition of these two characters in classical texts, often in structures where 对 introduced an object and 于 specified its scope or target, caused their boundaries to blur. Through repeated use, they gradually coalesced into a single, unitary word that syntactically and conceptually inherited 对's sense of "orientation toward" and 于's function of marking a thematic object or point of reference, resulting in the modern relational preposition meaning "regarding" or "with respect to".