Today at work I was doing some mildly boring and repetitive task, and naturally my mind began wandering the general direction of etymology. I remembered that during my last French course, near the end when the teacher was encouraging if not actually forcing us to speak nothing but French, I was trying to tell a story that involved the use of the word "advertisement". Now, the French do have a very similar word, "avertissement", but this is what's called in the language business a false friend: it looks like an exact match for a word in the target language but in fact has a very different meaning. In this case, "avertissement" means "warning"; the word I was looking for was "publicité". I kept saying "avertissement", she kept correcting me with "publicité", and eventually, I think, I got it right.
So; how does a word meaning "warning" in one language evolve into a word meaning "public notice of goods for sale" in another? Since I was at work, I couldn't look it up, so I listlessly ran it around in my mind. (I was listless because the air conditioning's broken and it's getting to be hot-humid-rainy season in these parts, hooray.) Stripping off the "-ise-", which turns a word into a verb, and "-ment", which turns it into a noun, leaves us with the Latin "advert". So great was my listlessness that I couldn't even decide what "-vert-" meant ("To steer? No...."), so I was reduced, while trying to keep at least part of my mind on the task at hand, to think of other words that had the same stem. "Convert", I thought. "Pervert". "Extrovert". Finally I remembered that "-vert-" and its various variants means "to turn", and that was the key to it.
"Advert" is a verb--though it isn't much used in English any more--which means "to turn attention [towards]". It is indeed from the Latin "ad-", "towards", and "vertere", "to turn" (which I've written about before). In the French version of the word, we warn someone about something by turning their attention to it; in the English version, we indicate that something is for sale by doing the same thing.