The audience watched open-mouthed as the magician pulled a rabbit from an empty hat.

open-mouthed 是由形容词 open（张开的）和名词 mouth（嘴）加上后缀 -ed 组成的复合形容词。字面意思是"嘴巴张开的"，引申为"惊讶得张大嘴的"或"目瞪口呆的"。