Even

Well, that thing happened again. One word led to another word which led to a big, broad clutch of words that I just flat-out don't have time to go into right now, so I'll deal with the easy bits first and the hard bits later.

To start: there is a most interesting article in the New Yorker (no surprise, as they run many interesting articles) by Jared Diamond (no surprise, as he's an interesting writer) about vengeance, and doesn't "vengeance" look like some other word? I thought it looked a bit like "vanquish", but that's ridiculous after a moment's thought, and naturally there is no connection. Then it occurred to me that "vengeance" seemed a fair bit like "vindictive", in both sense and sound, and...bingo!

"Vengeance" means "the infliction of retributive injury", and, as you can tell from the suffix, we got it from French: from "vengier", "to avenge", to be precise. Old French "vengier" came from, and seems very similar to, Latin "vindicare", "to vindicate", and this word is also, self-evidently, the source of "vindictive" (which may be viewed as "bitterly or vengefully vindicating").

Now, "vindicare" itself stems from "vindex", and no German window-cleaner jokes come to mind, but I'm sure they're out there. A vindex was a protector or avenger, and when you know that, it all falls into place.

The only missing piece is, as it so often is, the Indo-European root, "deik-", and you're either going to have to hunt it down yourself or wait until tomorrow when I pick it all apart for you. But trust me: you've never seen anything quite like it before. It's all over the map, this one.