to be dead set against something------完全反对（成语）

英文释义
(IDIOM) To be completely opposed to something.

例句
My grandfather is dead set against flying in airplanes, so we have to travel by train when we take him on a vacation with us.
我的祖父坚决反对坐飞机，所以带他去度假的时候，我们只能坐火车出行。