c. 1500, "consisting of money;" 1620s, "relating to money," from Latin pecuniarius "pertaining to money," from pecunia "money, property, wealth," from pecu "cattle, flock," from PIE root *peku- "wealth, movable property, livestock" (source of Sanskrit pasu- "cattle," Gothic faihu "money, fortune," Old English feoh "cattle, money").
Livestock was the measure of wealth in the ancient world, and Rome was essentially a farmer's community. That pecunia was literally "wealth in cattle" was still apparent to Cicero.
From Etymonline:
pecuniary (adj.)
c. 1500, "consisting of money;" 1620s, "relating to money," from Latin pecuniarius "pertaining to money," from pecunia "money, property, wealth," from pecu "cattle, flock," from PIE root *peku- "wealth, movable property, livestock" (source of Sanskrit pasu- "cattle," Gothic faihu "money, fortune," Old English feoh "cattle, money").
Livestock was the measure of wealth in the ancient world, and Rome was essentially a farmer's community. That pecunia was literally "wealth in cattle" was still apparent to Cicero.
Neat detail I wasn't aware of, thanks for sharing!