adverb, adjective · not good at; lacking proficiency in a specific area

他不善表达，在公开场合总是很沉默。

adjective · poorly managed or executed; unsuccessful; bad (of management, administration, or handling)

由于管理不善，这个项目宣告失败。

adverb · morally bad, ill, or wicked

他心怀不善，你要多加小心。

idiom · not to be underestimated; quite impressive or serious in nature

看他这身功夫，确实来者不善。