The compound adjective "water-filled" is a transparent combination of two native English elements. The first element, "water," descends from the Proto-Germanic watōr, a term of ancient Proto-Indo-European lineage (wódr̥) signifying the fundamental substance. The second element, "filled," originates from the Old English verb fyllan, meaning "to make full," itself from the Proto-Germanic fullijaną. The compound's logic is entirely literal and descriptive, directly stating the condition of a container or space being made full with water. It follows a common English pattern for creating adjectives that specify the contents of something, such as "sugar-coated" or "smoke-filled."

Structure: water (liquid) + filled (made full) - water: From Old English wæter, of Proto-Germanic origin; related to Dutch water and German Wasser. The noun denoting the clear, colorless liquid. - filled: The past participle form of the verb fill, from Old English fyllan, of Germanic origin; meaning "made full".