- The sash you wear  and the sash that
goes in a window  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).
The Hutchinson dictionary of word origins. 2013.