The word "耿耿" is formed by reduplicating the character "耿", a pictophonetic character where the left component "耳" (ear) suggests association with hearing or perception, and the right component "火" (fire) in its historical form indicates brightness, giving "耿" its core meaning of brightness or clarity. The reduplication emphasizes and intensifies this brightness, leading to an extended sense of something being vividly persistent or unresolved, hence its modern use to describe lingering feelings like anxiety or resentment, as seen in idioms such as "耿耿于怀".