be dead set against (someone or something)
To be completely opposed to someone or something. Most of the department is dead set against one of our peers becoming our boss, even temporarily. Many people in our small town are dead set against that big construction project, and I think you'll have a hard time changing their minds.
be dead set against something
If you are dead set against something, you are completely opposed to it. Her father was dead set against her marrying my father. I for one am dead set against charges for eye tests.