apropos apropos /ˌæprəˈpoʊ/

adjective

1. Of an appropriate or pertinent nature  ( apropos )
   of an appropriate or pertinent nature
   

adverb

1. Introducing a different topic  ( apropos , by the bye , incidentally , by the way )
   introducing a different topic
    Incidentally, I won't go to the party
   incidentally, I won't go to the party
   
2. At an opportune time  ( apropos , seasonably , well-timed , timely )
   at an opportune time
    Your letter arrived apropos
   your letter arrived apropos
   

preposition

1. () Concerning or related to a particular subject or matter.  ( apropos , re , as for , about , as regards , as to , in connection with , respecting , with regard to , concerning , regarding )
   Concerning or related to a particular subject or matter.