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.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.