dead set against
Completely opposed to, as in His parents were dead set against John's taking a year off from college. Set against has been used to mean “opposed to” since the 1400s. Dead acquired the meaning “utterly” in the 16th century.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.