ap ‧ ro ‧ pos /ˌæprəˈpəʊ, ˈæprəpoʊ $ -poʊ/ BrE AmE adverb formal
[ Date: 1600-1700 ; Language: French ; Origin: à propos 'to the purpose' ]
apropos of something used to introduce a new subject that is related to something just mentioned:
He had nothing to say apropos of the latest developments.
apropos of nothing (=not relating to anything previously mentioned)
Apropos of nothing, he suddenly asked me if I liked cats!