The Collaborative International Dictionary of English v.0.48:

'Snails \'Snails\, interj.
   God's nails, or His nails, that is, the nails with which the
   Savior was fastened to the cross; -- an ancient form of oath,
   corresponding to 'Od's bodikins (dim. of body, i.e., God's
   dear body). --Beau. & Fl.
   [1913 Webster]