From Latin 'fluēns' (flowing) — the metaphor is transparent: fluent speech flows like water, smoothly and without obstruction.
Able to express oneself easily and articulately in a language; smoothly graceful and effortless; able to flow freely.
From Latin 'fluentem' (accusative of 'fluens'), present participle of 'fluere' (to flow), from PIE *bhlew- (to swell, flow, overflow). The PIE root is also the source of English 'flood,' 'flow,' and 'fly' (originally meaning to move swiftly through air or water). The metaphor mapping water-flow onto speech is ancient: a fluent speaker's words 'flow' without obstruction or hesitation, just as an unimpeded river runs
Linguists distinguish several types of fluency. 'Productive fluency' is the ability to produce speech smoothly. 'Receptive fluency' is the ability to understand speech at normal speed. 'Reading fluency' is smooth, accurate reading with appropriate expression. A person can have high receptive fluency (understanding everything) but low productive fluency (stumbling when speaking) — a common stage in second-language acquisition. The metaphor