Self-deprecation is when you talk about your own skills or successes as if they were small or not worth much. You do it on purpose. Often it is a joke: someone who did well says, “Oh, I just got lucky,” or “I’m terrible at this,” when that is not really true. People do it to seem not too proud, to make others feel easy, or to get a laugh. Though actually it is not always light. Sometimes it is a habit, and the person has been putting themselves down for so long that it no longer sounds like a joke. The tidy idea is “being modest.” The messy truth is that the same words can be humour, manners, or a quiet kind of unhappiness — you have to hear the tone. The noun is a little formal; in speech people more often say someone is being self-deprecating.