The word 'tailwater' is a transparent compound formed within Modern English from two native Germanic elements. 'Tail', from Old English tægl, originally referred specifically to the rear appendage of an animal, a meaning that broadened metaphorically to denote the hindmost or concluding part of anything. 'Water', from the ubiquitous Old English wæter, stems from the Proto-Indo-European root wodr̥-/wed- (water, wet). In hydraulic and environmental engineering contexts, the logic of the compound is direct and spatial: the 'tail' water is the body of water found at the tail end or downstream section of a man-made structure or process, contrasting with the 'headwater' at the upstream source.

Structure: tail + water tail: From Old English tægl, tægel (the hindmost part of an animal). In this compound, it functions as a modifier meaning "rear" or "downstream". water: From Old English wæter (the clear liquid essential for life). In this compound, it functions as the core noun denoting the substance.