affluent:  The meaning ‘rich’ is a fairly recent development for affluent; it is first recorded in the mid 18th century. Originally the adjective meant simply ‘flowing’. It came, via Old French, from Latin affluent-, the present participle of affluere, a compound verb formed from the prefix ad- ‘towards’ and fluere ‘flow’ (the source of English fluid, fluent, flux, fluctuate, and many other derivatives). => fluctuate, fluent, fluid, flux
mid-15c., "flowing," from Middle French affluent (14c.) or directly from Latin affluentem (nominative affluens) "flowing toward, abounding, rich, copious," present participle of affluere "flow toward," from ad- "to" (see ad-) + fluere "to flow" (see fluent).