Definition of apropos preposition from the Oxford Advanced American Dictionary

 

apropos

 preposition
preposition
NAmE//ˌæprəˈpoʊ//
 
(also apropos of)
 
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concerning or related to someone or something Apropos (of) what you were just saying… Apropos of nothing.. (= with no connection to what was previously said or done)
See the Oxford Advanced Learner's Dictionary entry: apropos