1
general::
Phrase(s): dead set against someone or something
totally opposed to someone or something. • I’m dead set against the new tax proposal. • Everyone is dead set against the mayor.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms