apropos of
preposition
                                
              
          
                                                      : with regard to (something) : concerning                                      
              
                             
"I was always a fantasist," Nicholson says later, apropos of his childhood …— Ron Rosenbaum
Ron Rosenbaum
                                       Her comments about the weather were apropos of nothing. [=did not relate to any previous topic]
                         
                
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  Merriam-Webster unabridged




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