From Alan james – 3rd October 2020
A curious one. I think the most likely lead is SND sense III:
†III. adv. As in Eng. Comb. smacksmooth, adj., completely smooth and even, level, flush with the surface. Also in Eng. dial.; adv., smoothly, uninterruptedly, evenly.
which would imply that the field had been well cleared of stones, leaving it pretty well ‘smack-smooth’, with a clearance cairn in some out-of-the-way corner.
Otherwise, there’s an Older Scots sense ‘taste, scent’, I suppose a cairn might have had a smell, but I think it should have been investigated! Or a big, hearty kiss, which is in the spirit of Cuddle Cosy, but a cairn might be a rather conspicuous place for it. 😊