Meaning : Of an appropriate or pertinent nature.
Of an inappropriate or incorrectly applied nature.
malaproposMeaning : At an opportune time.
Example :
Your letter arrived apropos.
Synonyms : seasonably, timely, well-timed
Meaning : Introducing a different topic.
Example :
Incidentally, I won't go to the party.
Synonyms : by the bye, by the way, incidentally