foul own nest
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foul one's own nest
Fig. to harm one's own interests; to bring disadvantage upon oneself. (Alludes to a bird excreting into its own nest. See also It's an ill bird that fouls its own nest.) He tried to discredit a fellow senator with the president, but just succeeded in fouling his own nest. The boss really dislikes Mary. She certainly fouled her own nest when she spread those rumors about him.