1. prolonged speech without empathy or edification to the listener's subject matter; 2. conversation deemed inappropriate to a situation or the context of the discussion; 3. any conversation heard by a third party
"Yak, yak, yak. Is that all you do is talk?"
"She's yaking on the phone."
"He's yaking only to hear himself talk."
"Quit yaking and get back to work."
"He's just yaking. Off on a tangent. He doesn't know what he's talking about."