水清无鱼人贱无敌
[shuĭ qīng wú yú rén jiàn wú dí]
A play on a proverb which originally means "still water runs deep". Here it is modified to: "Clear water has no fish; low-class people cannot be beaten", implying being too clean or straightforward (water) leaves nothing hidden (no fish), or alternatively, that one's meekness or humbleness makes them invincible or unbothered.