sash

One sash comes from Arabic شَاش (šāš), meaning muslin cloth. The other comes from French châssis, meaning the frame of a window or door, which was mistaken for a plural, treated as 'sashes,' and trimmed down to 'sash' by the late 17th century.

The cloth sense stayed close to its source, naming a strip of fabric worn around the body. The window sense shifted through a grammatical accident: French châssis was misread as an English plural, so speakers peeled off the '-es' to make a new singular.

The window sense of sash is a doublet of chassis, both descending from the same French word châssis, just split apart by that odd plural mix-up.