The word "不分伯仲" is formed by combining the negative adverb "不" (not) with the verb "分" (to distinguish) and the two nouns "伯" (eldest brother, representing first or superior) and "仲" (second brother, representing secondary or inferior), which together denote a traditional ranking system. The phrase structurally negates the act of differentiating between these ranks, directly creating the meaning that two things are indistinguishable in quality or level, hence equally matched.