He admitted his mistake with a rueful smile, making people both pity and laugh at him.

rueful 是形容词，由名词 rue（懊悔、遗憾）加上形容词后缀 -ful（充满...的）构成，意思是"充满懊悔的"或"带着悔意的"。在这里形容一种既后悔又无奈的微笑表情。