The original dog in the manger was a character in a fable - a dog who insisted on sleeping in a manger and not letting the animals eat from it (see the full story here:
The Dog in the Manger - Wikipedia, the free encyclopedia ). The usual expression is just 'Don't be [such] a dog in the manger' - adding the adjective suffix strikes me as a bit odd. Maybe it's characteristic of some other dialect than mine.
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