Concerning or relating to a particular thing; at an opportune or fitting time; relevant or appropriate to what's being discussed.
From French 'à propos,' where 'à' means 'to/at' and 'propos' means 'purpose' or 'matter.' It literally means 'to the purpose,' and entered English in the 17th century as a phrase that got adopted as a single word.
You don't hear 'apropos' in casual speech anymore, but it lives on in the phrase 'apropos of nothing'—which means bringing something up that has no connection to the current conversation, a wonderfully specific phrase for when someone derails things.
Complete word intelligence in one call. Free tier — 50 lookups/day.