E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The cloth that covers the coach-box, in which hammer, nails, bolts, etc., used to be carried in case of accident. Another etymology is from the Icelandic hamr (a skin), skin being used for the purpose. A third suggestion is that the word hammer is a corruption of hammock, the seat which the cloth covers being formed of straps or webbing stretched between two crutches like a sailors hammock. Still another conjecture is that the word is a corruption of hamper cloth, the hamper being used for sundry articles required, and forming the coachmans box. The word box seems to favour this suggestion.