「まずい」 (Mazui) is an i-adjective meaning 'bad tasting' or 'distasteful'. It's used to describe food or drink that has an unpleasant flavor. While primarily about taste, it can also informally mean 'bad' or 'undesirable' in other contexts.

「まずい」は「bad tasting, distasteful」という意味(いみ)のい形容(けいよう)詞(し)です。食(た)べ物(もの)や飲(の)み物(もの)の味(あじ)が悪(わる)いことを表(あらわ)します。