"biased" 表示“有偏见的、偏心的”，"against" 指向“针对……的不利态度”，组合后强调对某个人、群体或事物持有不公平的负面看法或待遇（即因主观偏见而“偏向对立一方”）。
主语 + be 动词（am/is/are/was/were）+ biased against + 宾语（人/群体/事物）
接人/群体：针对特定个体或群体的偏见
She accused the manager of being biased against older employees.
（她指责经理对年长员工有偏见。）
Many studies show that hiring processes are often biased against women in STEM fields.
（许多研究表明，STEM领域的招聘流程常对女性有偏见。）
接事物：对某类事物的不公平否定
The judge was accused of being biased against environmental protection laws.
（法官被指控对环保法规有偏见。）
Some critics claim that the media is biased against conservative viewpoints.
（一些批评者称媒体对保守观点有偏见。）
be biased against：对……有偏见（负面）
be biased towards/for：对……有偏爱（正面）
例：The teacher is biased towards students who participate actively.
（老师对积极参与的学生有偏爱。）
简单来说，"be biased against" 就是“带着偏见反对/歧视某人/某事”，日常中常用来描述不公平的对待或看法。