Bite the Hand that Feeds You
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Bite the hand that feeds you, click on the image to watch the video “Bite the hand that feeds” by Nine Inch Nails
Meaning:
To attack someone who has helped you, to turn against someone who had done good things to you.
Origin:
This saying, since the 1700, has referred to an ungrateful dog which bit the hand of its owner in the very moment when it was being fed by him. This expression is now used to refer to those who act against you after you have spent time helping them (not literally feeding them).

Bite the hand that feeds you, click on the image to listen to the song, “Don´t bite the hand that feeds” by Modestep
Example:
-Oh, Gosh. I am so freaking mad.
-What happened to you Alfred?
-I helped John to get his new position as a manager and now he tells me I am fired! Can you imagine?
-Oh, my God. He has totally bitten the hand that used to feed him.