雕虫小技 (diāochóngxiǎojì): a trifling skill

From 《北史·李浑传》 (History of the Northern Dynasties, Biography of Li Hun): "雕虫小技，我不如卿；国典朝章，卿不如我" — "At the small art of carving insect-script, I am no match for you; at affairs of state and court ritual, you are no match for me." 虫 here originally referred to 鸟虫书, an ornate ancient calligraphy style — fancy decorative writing dismissed as a frivolous skill compared to statecraft.

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Often used as a self-deprecating disclaimer when showing off a skill — "oh, just a small trick" — though it can also be sarcastic when belittling someone else's flashy but shallow ability.