Slack-jawed is a term that describes someone with their mouth hanging open in a state of surprise or shock. However, there are a variety of synonyms that can be used to convey this same idea. Some examples include dumbstruck, agape, aghast, flabbergasted, stunned, and gobsmacked. Each of these words describes a person who is caught off guard, with their mouth gaping open in shock or amazement. Additionally, other words like speechless, thunderstruck, and stupefied could also be used to portray a similar idea. Ultimately, the choice of word depends on the context and the writer's personal style.