‘Làn shú’ literally means 'rotten熟'—a vivid, slightly humorous expression that emphasizes complete mastery through repetition and deep familiarity. Though the character 烂 (làn) usually means 'rotten' or 'spoiled', here it functions hyperbolically to suggest something so thoroughly internalized that it’s almost 'overcooked'—beyond mere memorization into effortless, instinctive command.

This adjective is commonly used to describe knowledge, skills, or routines that have been practiced so intensively they feel automatic: vocabulary lists, musical passages, mathematical formulas, or daily procedures like cooking a signature dish. It carries a positive connotation of expertise born from sustained effort—not innate talent—and often appears in contexts involving study, rehearsal, or habitual practice. While it can occasionally sound slightly literary or self-deprecating (e.g., joking about knowing something 'too well'), it’s widely understood and used across formal and informal registers.