He felt no resentment even when others failed to understand him.
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愠 is a literary word for quiet, restrained displeasure rather than open anger. The famous 《论语》 (Analects) line 人不知而不愠 ('not resentful when others fail to recognize you') makes it one of the best-known characters in classical Chinese.
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The heart radical 忄 plus a 温-like body: picture a heart that has gone warm - heated - under the surface, simmering rather than exploding. That simmering warmth is 愠, inner displeasure.
Supplies the sound. This is the same phonetic that gives 温 (warm) its reading; here the initial has shifted so the character is read yùn. It pictures a vessel under cover and carries no meaning in 愠.
