indubious vs ironclad

indubious

adj
  • Not dubious or doubtful; certain. 

  • Not doubting; unsuspecting. 

ironclad

adj
  • Solid or certain; not able to be disputed or questioned; irrefutable. 

  • Covered with iron, steel, or some metal; armor-plated. 

  • Rigorous; severe; exacting. 

  • Stubborn; inflexible. 

noun
  • An armor-plated warship, (especially) one preceding the invention of harveyized steel. 

  • A metal-plated ship, vessel, or vehicle. 

How often have the words indubious and ironclad occurred in a corpus of books? (source: Google Ngram Viewer )