slackjaw

An individual with loss for words, staring blankly with mouth open. Often clueless and/or confused.

Also used as two words: "slack jaw", "slack-jaw"

"When asked to calculate the tax, she stood there slackjawed."

"The slack-jawed prophet said nothing when asked about the future of man, thus answering the question with clarity"

"The Slack Jaw Prophets rocked the house"