L2 Lu4 ((exclusive))

I’m unable to determine the specific topic you mean by — this doesn’t clearly match a standard Mandarin pinyin phrase (e.g., lù for road, lǜ for green, lú for furnace) without tone marks or context. It could be a typo, a personal shorthand, or a niche term.

(Closing his eyes) Oh no...

Based on the standard curriculum for Chinese language learners (most commonly the textbook used in UK schools, or similar middle school Chinese programs), L2 LU4 typically refers to: l2 lu4

Used when an action is finished.