This is the site of a murder stone which, according to Wikipedia, is the only one documented in Scotland:-
'Murder stones are engraved stone monuments placed at or nearby the scene of a murder, often where crimes had caused particular shock or outrage within a community. The majority of stones known today were erected within a hundred year period, with the majority placed in the 1820s. One stone, a rough-cut boulder with the simple inscription "murder" is near Ardwell'
The book 'Tales of Galloway' by Alan Temperley gives a version of events which I've tried to paraphrase here:-
The Laird of Portcorkerie (McKinna) had a daughter who 'was wooed..by two suitors' (McDowall of Logan and Gordon of Castle Clanyard). All three lived within a few miles of each other in the Southern Rhins of Galloway. One day McDowall hears that Gordon had abducted the girl from her home and had taken her to Cardoness Castle (the home of Gordon's relatives) 50 miles away. Outraged, young McDowall jumps on his horse and heads for Cardoness to confront Gordon. 'Hot words were exchanged' but he rode away without the girl. A few miles from home at the site of the Murder Stone, McDowall was set upon and slain by a group of Gordon's followers. The saddled horse returned to Logan by itself and alerted the sleeping servants that their young master was missing. They went looking for him and found his body, but the assailants had vanished.
Time went by but Gordon failed to win over McKinna's daughter. In desperation, Gordon again resorted to violent measures: he took her father prisoner, tortured him and evertually killed him by throwing him off a nearby cliff. 'Thus, though he did not win the lady, young Gordon acquired her lands, which were at least part of what he so passionately desired'.
'There is no record that the murder of young McDowall was brought to law, but it did not go unavenged by the family. Shortly afterwards, Uthred McDowall, the young laird of Garthland...arranged the 'crewal slauchter' of James, the son of John Gordon of Barskeoch'
So, all in all, quite a horrific story! Wikipedia gives a different account ('Traditionally this writing dates to before 1551, when the Gordons obtained possession of the lands of Portercherie; at this point a McCulloch killed his rival, the laird of Castle Clanyard') - so if any budding historians can shed more light on the subject, I'd love to hear from them. Feel free to leave any further info (including dates) in the comments - or message me directly and I will update the description.
Please Note: The cache is not hidden in or near the drystone wall and there's no need to go into the garden of the house next door