Concerning, in connection with, as in Apropos of keeping in touch, I haven't heard from her in months. This idiom was a borrowing of the French à propos de ("to the purpose of") in the 17th century. At first it was used without of and meant "fitting" or "opportune," as in Their prompt arrival was very appropos. By the 1700s it was also being used with of, as in the current idiom, for "concerning" or "by way of."
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.