When is it appropriate to use the word "apropos"?
”Apropos” as an adjective means ”appropriate, pertinent or relevant”. (Have some apropos comments)
As an adverb ”apropos” means either ”at an opportune time” (Your letter comes apropos as usual) or ”appropriately or pertinently”. It also means “by the way or incidentally” (Apropos, your sister told me about your new job.)
As a preposition “apropos” is often used with “of” and means “with regard to” or “in respect of”, “regarding, concerning”. (His remark to Emerson apropos of diplomas).
Apropos is of French origin: "à propos" to the purpose
MOT Collins English Dictionary 2.0. Kielikone
Webster's third new international dictionary of the English language. Merriam, 1981.