She spoke unpleasantly to the cashier, which upset the people around her.

unpleasantly 是由形容词 pleasant（愉快的）加上否定前缀 un- 变成 unpleasant（不愉快的），再加上副词后缀 -ly 构成的副词，表示"不愉快地、令人不快地"。在这里形容说话的方式让人感到不舒服。