In the old days crofters would leave part of their crop unharvested for the hill folk [fairies]. These were the Picts, who had gone underground and were still putting up a bit of a resistance. They used to come at night and take the crop of the Norwegians or Vikings. This strengthened the superstition and people would leave a lamb out for them. Later this extended to leaving a part of the croft unworked so that the hill folk could deal with it as they liked. This was known as the Deil's [devil's] Portion; it consisted of a number of the least valuable rigs [arable fields].
This happened a long time ago, about the time the Norse occupation was ending and the Scotch influence came back in.