Old English rice "strong, powerful; great, mighty; of high rank," in later Old English "wealthy," from Proto-Germanic *rikijaz (source also of Old Norse rikr, Swedish rik, Danish rig, Old Frisian rike "wealthy, mighty," Dutch rijk, Old High German rihhi "ruler, powerful, rich," German reich "rich," Gothic reiks "ruler, powerful, rich"), borrowed from a Celtic source akin to Gaulish *rix, Old Irish ri (genitive rig) "king," from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule" (compare rex).
The form of the word was influenced in Middle English by Old French riche "wealthy, magnificent, sumptuous," which is, with Spanish rico, Italian ricco, from Frankish *riki "powerful," or some other cognate Germanic source.
Old English also had a noun, rice "rule, reign, power, might; authority; empire." The evolution of the word reflects a connection between wealth and power in the ancient world. Of food and colors, from early 14c.; of sounds, from 1590s. Sense of "entertaining, amusing" is recorded from 1760. The noun meaning "the wealthy" was in Old English.