c.1500, from Latin pecuniarius "pertaining to money," from pecunia "money, property, wealth," from pecu "cattle, flock," from PIE root *peku- "wealth, movable property, livestock" (cf. Sanskrit pasu- "cattle," Gothic faihu "money, fortune," Old English feoh "cattle, money").
Livestock was the measure of wealth in the ancient world. For a possible parallel sense development in Old English, see fee, and cf., evolving in the other direction, cattle. Cf. also Welsh tlws "jewel," cognate with Irish tlus "cattle," connected via notion of "valuable thing."