滾瓜爛熟

1. • lit. ripe as a melon that rolls from its vine (idiom) fig. to know fluently
2. • to know sth inside out
3. • to know sth by heart

Character by character

• 滾(gǔn)boil
• 瓜(guā)melon
• 爛(làn)soft
• 熟(shú)ripe