[zfxfr]

Ohh I always thought "apropos" was from the French expression "à propos (de)... " which means "about". I naïvely believed that the person who coded this command must be French.

[killerbat00]

The English usage is borrowed from French![0] However, it doesn't look like the original author was French[1]. I shouldn't be, but I am quite surprised to see that the apropos command as we know it is 43 years old!

[0] https://en.wiktionary.org/wiki/apropos [1] https://www.mankier.com/1/apropos.mandoc#History

[ochrist]

It's an ordinary word in Danish, meaning 'about something'. It can be used at the start of a sentence. Example: "Apropos vegetables I hate tomatoes".

[Isamu]

You are correct about the origin except it came from French into English long ago.