To be ungrateful and turn against one's benefactor, repaying kindness with enmity; to show extreme ingratitude.
他曾经受过那位老先生的许多帮助，现在却做出这种忘恩负义的事情，真是鸮鸟生翼
He had received a lot of help from that old gentleman, but now he's doing such an ungrateful thing; it's truly a case of 'the owl growing wings'.
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