What is the Meaning of the Expression
SOMEONE’S MOUTH FELL OPEN?
This means someone was very surprised.
The idea is that when someone is very surprised, her bottom jaw (the bottom half of your mouth) drops down.
Note: This is similar to the expression s.o.’s jaw dropped
1. When they told her grandmother they were getting married in six weeks, her mouth fell open.
2. When she told me the news, my mouth fell open. I couldn’t think of a thing to say to her.
Note: Replace someone with a possessive adjective such as: my, your, his, or her