An idiom meaning that something causes a hundred harms and not a single benefit — utterly disadvantageous or having no redeeming features.
あんなやり方は短期的に見ても百害あって一利なしだ。
That approach is completely disadvantageous even in the short term — all harm and no gain.
タバコは健康にとって百害あって一利なしと言われている。
People say smoking is all harm and no benefit for your health.