Alternate Scot-speak for "cairn".
If you're Scots you can't leave well enough alone, or you simply want to be left alone, so you invent your own word and then build a pile of stones and hide behind it, daring anyone to come by and bug you.
If they do, after all that, you can bury them in a hail of rocks (which you now have an ample supply of, since you have thoughtfully prepared against just such an eventuality).
This gives the whole concept of "prickly" a new meaning.
So a carn is either a pile of stones to show the way, a supply of stones laid in against busybodies, or a tomb marking the last known location of a busybody, now only a body and no longer quite so busy.
This might also explain why the Scots' national flower is the thistle.
Kinda fits, don't it?