Their jaws is dropping - Idioms by The Free Dictionary
(one's) jaw drops (redirected from their jaws is dropping)
(one's) jaw drops
One shows utter shock or great surprise, especially upon learning unexpected news. Our jaws positively dropped when we found out how much they were paying in rent. Oh my gosh, I can't believe you're going to have a baby! My jaw is dropping!
your ˈjaw drops your mouth opens because you are very surprised: When they told her that she had won a million dollars, her jaw dropped in amazement. ▶ ˈjaw-dropping adj.: Taking the desert road was a jaw-dropping experience.