E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The Thersits of Fingal; brave even to rashness.
Blow for blow or claw for claw, as Conan said. Conan made a vow never to take a blow without returning it; when he descended into the infernal regions, the arch-fiend gave him a cuff, which Conan instantly returned, saying Claw for claw.
Blow for blow, as Conan said to the devil.Scott: Waverley, chap. xxii.