the same meaning as "jaw dropped" - shocked "...eyes wide open and jaw dropped" A more common expression would be 張大嘴巴，or 張著嘴巴

每個人都睜大雙眼，張著下巴 Everyone is glaring and mouths agaped.

It means keep mouth open.就是张大嘴巴，形容很吃惊，震惊。

the lower jaw，下巴。 張著下巴，通常指看到讓人驚訝、目瞪口呆、或者無語的事情所作出的反應。