罄竹难书 is a four-character idiom (成语) meaning 'so numerous that even using up all the bamboo slips would be insufficient to record them'. It's exclusively used to describe crimes, misdeeds, or wrongdoings that are too many to count — never for neutral or positive things. The literal image comes from ancient Chinese writing on bamboo slips: 'exhaust the bamboo, hard to write'. You'll encounter it in formal or literary contexts, especially news reports and historical writing.