I don't know for sure. I am pretty sure I did not learn it in school. I read German press daily from the 1970s to the 1990s, less frequently thereafter. So I guess that's the origin. I have never been interested in linguistics as a science. I speak 4 foreign languages, so I have some experience in comparing languages.
A sibling comment says that indo-germanic remains a standard scientific term in German, there is no nationalist background. I cannot comment on the correctness of that claim, but it would explain why I used it. German press is not suspect of nationalism, quite the opposite. (There are of course exceptions, but I have never read those. I am part of the pre 1990 generation that feels uncomfortable when seeing German flags flown by individuals in public.)
> A sibling comment says that indo-germanic remains a standard scientific term in German, there is no nationalist background.
I replied to the sibling about why I think the English word has strong nationalist connections.
I don't know about its German equivalent (Wiktionary claims it's a synonym for Indo-European), but this discussion thread is in English, and furthermore concerns linguistics, and it that context, it has nationalistic connections.
Even the Internet thinks so: the second link on google when you search for the term is a Wikipedia article about the nationalist myth it represents. What appears when you search for it in German?
I understand you didn't use it nationalistically, of course. Sometimes things good and bad are both lost and gained in translation.
If you search for "indogermanisch" you're going to get the Wikipedia article canonically named "indogermanische Sprachen" first. The second result I see is the English Wikipedia article called "Indo-European languages" and the rest of the articles also appear to be very scientific.
Maybe the term has some weird connotations in English, but that's certainly not true everywhere and it's also not necessarily true in linguistic discourse because English only became relevant as a scientific language relatively recently (German and French used to be much more common) and there's still to this date a lot of linguistic research being published in languages other than English (e.g. why would somebody who researches the German language publish in English?).
> Maybe the term has some weird connotations in English, but that's certainly not true everywhere and it's also not necessarily true in linguistic discourse because English only became relevant as a scientific language relatively recently (German and French used to be much more common).
This discussion is in English though, not German or French.
I only know it from studying about how the Nazis appropriated and twisted the legitimate field of historical linguistics to support their objectives.
I've only heard it used to describe that ideology, or sadly, by acquaintances who subscribed to portions of that ideology.