The word 忠心耿耿 is formed by combining the morpheme 忠心, meaning "loyal heart" (from 忠 "loyal" and 心 "heart"), with the duplicated morpheme 耿耿, which originally denotes "bright" or "shining" but here functions as an intensifier to emphasize steadfastness and unwavering commitment, thus constructing a compound adjective that conveys extreme loyalty.