Etymology of "amoral"

Many internet sites (like this one) say that the word amoral was coined by Robert Louis Stevenson (1850-1894) as a differentiation from immoral. These sites also say that amoral comes from the Greek privative prefix a- "not" and the Latin word moral.

If this were true, amoral and immoral would have completely equivalent meanings, but in philosophy the term amoral implies not to be concerned with morality, whereas immoral implies a direct opposition to certain standards of morality.

So my hypothesis is that the word amoral comes from the Latin prefix a- or ab-, meaning "away from" (source). This makes much more sense to me both for meaning and for coherence with moral, which is also Latin.

I am not an expert, so could you please tell me your opinion about this?

The word occurs in one of Stevenson's essays. He is reflecting on the ways in which a writer may choose to approach the subject of his writing:

"There is a vast deal in life and letters both which is not immoral, but simply a-moral; which either does not regard the human will at all, or deals with it in obvious and healthy relations; where the interest turns, not upon what a man shall choose to do, but on how he manages to do it; not on the passionate slips and hesitations of the conscience, but on the problems of the body and of the practical intelligence, in clean, open-air adventure, the shock of arms or the diplomacy of life".

Today, 'amoral' often has connotations of being unable to tell right from wrong, but this is clearly not what Stevenson had in mind. He was simply drawing a distinction between those actions that have an obvious moral dimension and those that either lack this dimension or where (speaking as a writer) this dimension is of secondary importance.

So the meaning of a word is liable to change with time, or the word acquires new senses; and as John Lawler has pointed out, a word borrowed from another language cannot be relied on to mean what it meant in its parent language, nor can its meaning necessarily be derived from etymological analysis. The latter is at best a guide to the meaning, not an infallible marker of it.