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To talk incessantly and/or at great length. I became so bored while the professor jawed away at us that I nearly fell asleep in the middle of the lecture. Whenever I meet up with Tammy, our "conversation" is always just her jawing away while I listen patiently on.
1. With one's mouth hanging open, as due to surprise or dim-wittedness. I had to clear away the people who were standing slack-jawed on the side of the road, staring at the car wreck.
2. Moronic; very slow or dim-witted. I tried asking for directions at the gas station, but there was just some slack-jawed yokel working behind the counter.