act of ingratitude; ungrateful act
A formal or literary term for an action that shows a lack of gratitude, often implying a betrayal of kindness received.
I was truly disappointed by his ungrateful act.
忘恩 is the abstract concept of ingratitude, while 忘恩行為 refers specifically to an act or deed that embodies that ingratitude.
Compound of 忘恩 (ingratitude) and 行為 (act, deed). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning.