There's a bit of a military theme with this spot. Maybe that's no surprise if it's on a road following the Annan valley, heading north-south? An RCAHMS report from 1920 says the "stone is said to have been erected to mark the spot where Prince Charlies' troops bivouacked on their march into England in the '45." That's the 1745 uprising led by 'Bonnie Prince Charlie' 'f course. Though confusingly, this book thinks that it was actually another Charles, the English Charles the second, who stopped here for breakfast with his army, in 1651. And according to another contemporary story* "there is a house called Powdine..; that house hath been haunted these fifty or sixty years.. I spoke with the [owner].. He told me many extraordinary relations consisting in his own knowledge; and I carried him to my master, to whom he made the same relations - noises and apparitions, drums and trumpets heard before the last war; yea, he said, some English soldiers quartered in his house were soundly beaten by that irresistible inhabitant.." And Not that I'm saying the stone is related to this folklore (although the house is just across the road) but it's got a similar theme?
*From 'Laws' Memorials', quoted in 'Witchcraft and superstitious record in the south-western district of Scotland' by JM Wood (1911).
An entry from Ancient Stones, an online database that covers most of the standing stones, stone circles and other stones found in South East Scotland. Each entry includes details, directions, photograph, folklore, parking and field notes on each location.