The quality of being nasty, unpleasant, or cruel — and also a single act of that kind. Both uses are normal: 'There was a lot of unkindness in her voice' (the quality, uncountable), and 'I have never forgotten his unkindness to my mother' or 'small unkindnesses' (acts, countable). What makes this word useful is its size. It is smaller than 'cruelty'. Cruelty makes us think of real harm done on purpose. Unkindness covers the ordinary, everyday damage people do to each other: leaving someone out, a comment that was true but did not need saying, not answering a message, laughing at the wrong moment. That is why it is such a common word in schools, in families, and in writing about how people treat one another. It names something too small to call cruelty but too real to call nothing. It is a slightly careful, considered word — people use it when they are naming a pattern of behaviour rather than shouting about one incident. 'I was surprised by his unkindness' sounds like a judgement someone arrived at, not a burst of anger. One last thing, for fun, because your students always like it: a group of ravens is called 'an unkindness of ravens'. It is a very old, half-playful collective noun and nobody uses it in real life, but it shows up in poems and games and film titles.

刻薄、不友善（的言行）；比 cruelty（殘忍）輕，指日常那種讓人受傷的小舉動