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. For a possible parallel sense development in Old English, see fee, and compare, evolving in the other direction, cattle. Compare also Welsh tlws "jewel," cognate with Irish tlus "cattle," connected via the notion of "valuable thing," and, perhaps emolument.
An earlier adjective in English was pecunier (early 15c.; mid-14c. in Anglo-French), from Old French; also pecunial (late 14c.).
"in the matter of, in the (legal) case of," c. 1600, probably from Duns Scotus; Latin, from re, ablative of res "property, goods; matter, thing, affair," from Proto-Italic *re-, from PIE *reh-i- "wealth, goods" (source also of Sanskrit rayi- "property, goods," Avestan raii-i- "wealth").
Roman god of the underworld, early 14c., from Latin Pluto, Pluton, from Greek Ploutōn "god of wealth," from ploutos "wealth, riches," probably originally "overflowing," from PIE root *pleu- "to flow." The alternative Greek name or epithet of Hades in his function as the god of wealth (precious metals and gems, coming from beneath the earth, form part of his realm). The planet (since downgraded) was discovered 1930 by U.S. astronomer Clyde W. Tombaugh; Minerva also was suggested as a name for it. The cartoon dog first appeared in Walt Disney's "Moose Hunt," released April 1931.