The word "半斤八两" is formed by directly combining two quantitative phrases: "半斤" (half a catty) and "八两" (eight taels). It originates from the historical Chinese weight system where one catty equaled sixteen taels, making half a catty precisely equivalent to eight taels. The phrase's structure juxtaposes these equal measurements to convey a literal equivalence, which then serves as the basis for its idiomatic meaning indicating two things are nearly identical.