Sentence:  And it is symptomatic of the many paradoxes of Lederer's life that of all the people in the room, Brotherhood is the one whom he would most wish to serve, if ever he had the opportunity, even though — or perhaps because — his occasional efforts to ingratiate himself with his adopted hero have met with <WSD>iron</WSD> rebuff. 
Meaning of the word marked with <WSD> tags:  Strong (as of will), inflexible.