最坏(zuì huài) - worst (most bad) in Mandarin

最坏 (zuì huài) is 最 (zuì), "most", plus 坏 (huài), "bad" - the superlative of bad, worst. It is the natural opposite of 最好 (zuì hǎo), "best", and the two sit either end of the same scale.

做最坏的打算 (zuò zuì huài de dǎsuàn), "to make the worst-case plan", is a fixed, very common phrase for preparing for the worst outcome before it happens - the Mandarin equivalent of "hope for the best, plan for the worst". 最坏的情况 (zuì huài de qíngkuàng), "the worst-case situation", works the same way.

最坏 (zuì huài) leans toward badness in a general, often moral or circumstantial sense - the worst day, the worst outcome, the worst possibility. Where a result is being ranked on quality or performance rather than on how bad the circumstances are, 最差 (zuì chà) is the more natural choice; see that page for the distinction.