sash

The sash you wear [16] and the sash that goes in a window [17] are distinct words. The former comes from Arabic shāsh ‘turban’, and that is exactly how English first acquired it: ‘All of them wear on their heads white shashes and turbans, the badge of their religion’, George Sandys, Travels 1615. But the Arabic word also denoted a strip of muslin or other material from which such turbans were constructed, and it is that application that led towards the end of the 17th century to the current sense of the English word. The altered form sash appeared around the same time. Sash ‘window-frame’ was originally chassis, an early borrowing of French chassis ‘frame’ (it was acquired again in the sense ‘frame of a carriage’ in the 19th century). This evolved to shashes, and in due course came to be regarded as a plural form, so a new singular sash emerged. French chassis itself goes back ultimately to Latin capsa ‘box’, source of English capsule, case, etc. => CAPSULE, CASE, CHASSIS

* * *

   A sash window is one that slides up and down (as distinct from a casement window, which opens on hinges). The sash is the window's frame. The word evolved as a singular form of sashes, itself a form of shashes, adopted from the French singular word chassis, 'frame' (now modern English chassis).