From the Aberdeenshire Ordnance Survey Name Books (1865-71).
This name is given to a standing stone 7 chains south west of Roddentree. There seems to have been a stone circle as other stones are beside this one, but are deeply imbedded in the earth. Many years ago the word ‘Cummer’ was applied to the Mistress of the house, latterly the meaning changed to ‘a worthless woman’. There are traces of a house near this stone which was inhabited by an old woman, many years ago. Hence the name.
I had a look in the Oxford English Dictionary and it has various versions of this Scottish word: a godmother, a female companion, a gossip, or a familiarly-applied word for a woman “with various local specific applications, e.g. young woman, lass, girl, witch, wise-woman, midwife, etc.” (Nothing particularly derogatory unless you mean it that way I suppose).