"Joe Biden Just Called a Woman at One of His Events a 'Lying, Dog-Faced Pony Soldier.' What?"

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My question exactly, and Slate (Matthew Dessem) has what might well be the answer, though it still draws a blank on "dog-faced." (Rattlesnake-tongued might be seen as referring to lying, perhaps because of the forked tongue, but dog faces seem pretty honest to me.)