Kain bairns or cain bairns were infants who, according to Scottish superstition, were seized by warlocks and witches, and paid as a tax or tithe to the Devil. John Jamieson is in error when he derives the term from the Scottish Gaelic ceann meaning a "head". Càin is in fact a Gaelic word for a tribute, tax or tithe, and is the origin of the Lowland Scots term. "Bairn" means a child. The word was in use in the Scottish Borders, according to Walter Scott's Minstrelsy of the Scottish Border.
It is unconnected with Cain in the Bible.