The word 'windowsill' is a transparent compound formed from two native English elements. 'Window' itself has a poetic Norse origin, literally meaning 'wind-eye', referring to an opening for light and air. 'Sill' is an ancient Germanic word for a foundational beam or threshold. When combined, the term logically denotes the specific horizontal ledge forming the base of a window opening. Its formation follows a straightforward pattern in English where two concrete nouns are joined to specify a particular architectural feature, moving from the general concept of a 'sill' to the precise one belonging to a 'window'.

Structure: window + sill - window: From Old Norse 'vindauga' ('wind eye'), a compound of 'vindr' (wind) and 'auga' (eye). In this compound, it functions as the modifying noun specifying the type of sill. - sill: From Old English 'syll' or 'sylle' (beam, threshold, foundation). It is a Germanic base meaning a horizontal supporting member or ledge. In this compound, it functions as the head noun, the core object being described.