To be ungrateful and disloyal; to forget favors and turn against one's benefactor; to betray one's obligations.
那个受过他大恩的人，如今却负恩忘义，在他最需要帮助的时候袖手旁观。
The person who had received great kindness from him now proved ungrateful and disloyal, standing by when he most needed help.
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