Apropos is used as a preposition to mean "with regard to." It is frequently used in the phrase "apropos of."

// Sean interrupted our conversation about politics and, apropos of nothing, asked who we thought would win the basketball game.

As an adjective, apropos describes something that is suitable or appropriate, as in "an apropos nickname."

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"Once, at the height of COVID, I dropped off a book at the home of Werner Herzog. I was an editor at the time and was trying to assign him a review, so I drove up to his gate in Laurel Canyon, and we had the briefest of masked conversations. Within 30 seconds, it turned strange. 'Do you have a dog? A little dog?' he asked me, staring out at the hills of Los Angeles, apropos of nothing. He didn't wait for an answer. 'Then be careful of the coyotes,' Herzog said." — Gal Beckerman, The Atlantic, 8 Jan. 2025

Apropos wears its ancestry like a badge—or perhaps more fittingly a beret. From the French phrase à propos, meaning "to the purpose," the word's emphasis lands on its last syllable, which ends in a silent "s": \ap-ruh-POH\. Apropos typically functions as an adjective describing what is suitable or appropriate ("an apropos comment"), or as a preposition (with or without of) meaning "with regard to," as in "apropos (of) the decision, implementation will take some time." The phrase "apropos of nothing" is used to signal that what follows does not relate to any previous topic.