小时了了,大未必佳 is a Chinese idiom (chengyu) meaning “a child prodigy does not necessarily become a great adult”. Early brilliance does not guarantee future success. Literally, “Clever in childhood, not necessarily excellent as an adult”.
During the Eastern Han Dynasty, 10-year-old Kong Rong went to visit Li Yuanli, the famous governor of Henan, claiming to be a relative. Li Yuanli didn't recognize him, so Kong Rong cleverly linked the descendants of Laozi and Confucius as relatives to establish their connection.
Chen Wei, a Grand Master, mocked Kong Rong, saying, 'Clever in childhood, but not necessarily outstanding as an adult.'
Kong Rong immediately retorted, 'I imagine you must have been very clever in your childhood too!'