Definition of ironclad oath in English:

ironclad oath

noun

  • (In the United States) an oath introduced during the Civil War of 1861–5 requiring certain government officials, employees, and members of certain other groups to swear that they had been loyal to the United States government in the past and would continue to be loyal in the future (now historical); (in extended use) any oath characterized by the severity of its requirements and penalties.

Origin

Mid 19th century; earliest use found in The New York Times.