The word 'taillight' is a transparent English compound formed in the modern era with the advent of automobiles. The first element, 'tail', draws from its Old English root meaning the rear extremity, a concept easily transferred from animals to vehicles. The second element, 'light', originates from the common Germanic root for brightness. The compound's logic is purely descriptive, literally meaning "a light at the tail/rear," which directly and accurately reflects its function and position on a car, bicycle, or other vehicle.

Structure: tail + light tail: From Old English 'tægl', 'tægel', meaning the hindmost part of an animal. In this compound, it functions to specify location (the rear part). light: From Old English 'lēoht', meaning brightness or a source of illumination. In this compound, it functions as the core object (the illuminating device).