It is my experience that the word ‘docile’ is one belonging primarily to animals. Its most common usage is the phrase ‘a docile creature,’ and when I, at least, use the word of people instead of animals, it operates by analogy of this sense, not independently, and applies animal characteristics to a human being. To me the word ‘docile’ suggests tameness and moreover a gentleness bordering almost on indifference. It is not so moral as 'lazy' nor so cold as 'inactive:' it is an animal word. That this meaning I have inherited is at odds with etymology does not surprise me; that I found it absent from every dictionary I have consulted does.
‘Docile’ is a direct borrowing of the Latin docilis, which means, quite literally, ‘teachable;’ it is an adjective formed from the verb docere, 'teach,' the same Latin root which gives us doctor, doctrine, and docent, among other words. If you look up ‘docile’ in an English dictionary, you will not find a definition far removed from this origin: the standard definition, which I have found in roughly the same form in several sources, contains two meanings, first, "willing to be taught or teachable," and second, "yielding to instruction, obedient, or tractable." I was surprised to encounter the first meaning, with which I am not at all familiar, because it was so exactly close to the Latin; the second gave me pause for being so slightly derived, and for maintaining a similar distance from what I would have made the current meaning. And in neither did I find even the slightest "esp. of animals" to vindicate me.
Yet it is not hard to see the shift from the Latin meaning of ‘docile’ to the one that is now colloquial. An animal which could be described as docile would probably also be described as tame or gentle, and if one were exposed only to phrases combining 'docile' with such words, it would make sense to take it for their synonym—and how else is meaning diluted but through the ingrained formulas of this process?—. Were I pressed, I would guess it began as something of a technical veterinarian or taxonomist’s term, perhaps even in its Latin original, and gained only later a new generality out of that specific use. What I find the last irony is that a word which had so humane a meaning, a word about learning and education, has become a word passing almost for an inborn quality in beasts. I leave you, then, with this dictum of Quintilian, from the second book of his Institutio Oratoria (“Oratorical Education”), a memory of the old docile: Nam ut illorum officium est docere, sic horum praebere se dociles, “For as it is the duty of instructors to teach, so it is the duty of students to prove themselves easily taught.”