Latin mel — honey. Latin fluere — to flow. The word means, with no metaphorical distance at all, honey-flowing.

And it started literal. In the early 15th century it entered English meaning sweet as honey, sweetly or smoothly flowing, and was applied to odours as readily as to speech. The physical sense stayed live for centuries: Milton, in Paradise Lost (1667, Book V), writes of brushing mellifluous dews — actual moisture, actual sweetness, no voices anywhere in sight.

The restriction to sound is a narrowing, and a recent one by the word's standards.

Which leaves a small double edge in the modern use worth noticing. Mellifluous describes how something flows, not what it contains. A mellifluous speaker is easy to listen to — and the word says nothing whatsoever about whether they are saying anything. Used of prose, it can be a compliment about rhythm and a very quiet complaint about substance, and skilled reviewers have used it in exactly that double register for a long time.

The fluere half has other descendants worth knowing, because they explain the sensation. Fluent. Fluid. Influence. The same root that names a smooth voice names the flowing of one thing into another — which is what a genuinely mellifluous speaker does to a listener.

One word to keep separate, because they are confused constantly. Melliferous — one letter different — means honey-producing: bearing nectar or pollen. A flower is melliferous. A baritone is mellifluous. One end of the process makes the honey; the other pours it.

And the word's most distinguished bearer: Bernard of Clairvaux (1090–1153), known for centuries as the Mellifluous Doctor for the sweetness of his writing — a title formalised in a 1953 papal encyclical named Doctor Mellifluus, published on the eight hundredth anniversary of his death.