Simplified form of 囱. Derived from the top component of 悤. Depicts a line piercing an object. The meaning later shifted to "window" (now written 窗) and "chimney".

Evolution

Seal script

Shuowen (~100 AD)

Seal script

Shuowen (~100 AD)

Seal script

Shuowen (~100 AD)

Regular script

Modern