Moving back and forth quickly, usually side to side (like a dog's tail), or gossiping.
From wag (to move back and forth), which comes from Middle English wagen, possibly from Old Norse. The word was always onomatopoetic-sounding, mimicking the motion.
A wagging tongue and a wagging tail use the same word because both are involuntary ways animals express emotion—this is why 'wagging tongues' gossip seems as natural as a happy dog's tail.
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