Tā kǎo le dìyī míng, biàn kāishǐ zìmíng-déyì, bùzài nǔlì xuéxí.
After coming first, he became smugly self-satisfied and stopped studying hard.
Tips
memory
自 (self) + 鸣 (make a sound / crow) + 得意 (proud / pleased). Picture a rooster crowing about its own achievements - crowing at itself. That smug self-congratulation is the heart of this idiom.
