The word pecuniary ("of or pertaining to money") was borrowed in 1506 from the Latin word pecunia, which meant "money". That's from the earlier word pecus, or "cattle"; the connection is that livestock was a measurement of wealth in Ancient Rome. Pecus traces to the Proto-Indo-European root peku, which had the same definition. Coincidentally, that same word was also borrowed into Proto-Germanic as fehu (because of Grimm's Law, which made voiceless stops become voiceless fricatives, the p became an f and the k became an h). That turned into Old English feoh, which also meant "wealth in livestock", and by way of a little confusion with the Old French word fief ("possession"), it in turn mutated into fee, the word we use to describe monetary transactions today.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.