other man
noun
                                
              
          
                                                      : a man with whom a married woman has an affair                                      
                
                  —usually used with the
    
              
                             
E.G. Marshall plays a man who, believing himself cuckolded, plants a bomb in his own basement to destroy the Mrs. and the other man.— Mike Flaherty
Mike Flaherty
                         
                
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  Merriam-Webster unabridged




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