“Claw for claw,” as St Conan said to the devil. The expression “blow for blow” occurs in Waverley, and in a note the following explanation is given of it.
In the Irish ballads relating to Fingal, or Fion, there occurs, as in the primitive poetry of most nations, a cycle of heroes, each of whom has some distinguishing attributes. Upon these qualities and the adventures of those possessing them many proverbs are formed which are still current in the Highlands. Amongst other characteristics Conan is distinguished as in some respects a kind of Thersites, but brave and daring even to rashness. He had made a vow that he would never take a blow without returning it, and having like other heroes of antiquity descended into the infernal regions he received a cuff from the Arch-fiend who presided, which he instantly returned, using the expression in the text. Sometimes the proverb is rendered thus–‘Claw for claw, and the devil take the shortest nails.’
We should be very unwilling to believe that St Conan and Thersites–the evil-minded, “scurrilous Grecian”–had anything in common, and though in those rough early days even a churchman–with little law to look up to or to help him–might now and then have to take it into his own hands, he could not well be a brawler, and at the same time retain the reputation for piety which we know was attached to St Conan. The Conan of the ballad of Fion may have been a Thersites, and the saying may have originated in his time, and may have been appropriated and applied to their master by the monkish scribes. At anyrate, one of them gives the following explanation of it: It appears that at one period of the saint’s earlier life the Evil One had great power in Argyllshire. We find in everyday life that one man, when disputing with another, will now and then find it politic to bargain and perhaps give way a little, even when he knows himself to be in the right, rather than provoke a contest in which he is not sure he will altogether be the victor, and so the good monk found it necessary to temporise with the Devil. There were many very bad characters–so says the old chronicler–in those days in the district of Lorn, or what we call Lorn now, to whom St Conan could not altogether deny the Fiend a right; some of whom were hopelessly wicked, and the latter was about sweeping them all, middling, bad and very bad, into his net. St Conan gave up the last and offered to draw alternately for the others, stating his determination if this proposal was refused of fighting most desperately for them all. The Devil, knowing how very formidable an opponent the saint would prove, agreed. The very black ones were raked away, and then the champions took in turn the souls of the remainder. It was while they were thus engaged that the saint made use of the memorable expression, for his great enemy grew so terribly excited in the grim game that he could not keep his turn, and was continually stretching out his awful hands for his prey. “Keep your turn,” thundered the saint, “play fair, claw for claw.”
— The Highland Monthly, Vol. II, no. 18, September 1890.