The compound adjective 'sun-filled' is a transparent modern English formation. Its first element, 'sun', descends from the Old English sunne, a term with ancient Proto-Indo-European ancestry shared by words for the sun across many languages. The second element, 'filled', originates from the Old English verb fyllan, meaning to make full, which is directly related to the adjective 'full'. The compound logically combines these two concepts, creating a descriptive term that literally means 'made full of sunlight'. This straightforward morphological process is common in English for creating vivid imagery, where a noun is paired with a participle to denote a state or quality, evolving from the literal idea of a space being saturated with light to describe any bright, cheerful, or luminous atmosphere.

Structure: sun (the star) + filled (made full) - sun: From Old English sunne, of Germanic origin; related to Dutch zon and German Sonne, from an Indo-European root shared by Greek hēlios and Latin sol. The noun referring to the celestial body. - filled: From Old English fyllan, of Germanic origin; related to full. The past participle of the verb 'fill', meaning 'made full'.