Is it ensure, insure, or assure?
1580s, "flowing freely" (of water, also of speech), from Latin fluentem (nominative fluens) "lax, relaxed," figuratively "flowing, fluent," present participle of fluere "to flow, stream, run, melt," from PIE *bhleugw-, extended form of *bhleu- "to swell, well up, overflow" (cf. Latin flumen "river;" Greek phluein "to boil over, bubble up," phlein "to abound"), an extension of root *bhel- (2) "to blow, inflate, swell;" see bole. Used interchangeably with fluid in Elizabethan times. Related: Fluently.