The Collaborative International Dictionary of English v.0.48:

Slug \Slug\, v. t. [imp. & p. p. Slugged; p. pr. & vb. n.
   Slugging.]
   1. To load with a slug or slugs; as, to slug a gun.
      [1913 Webster]

   2. To strike heavily. [Cant or Slang]
      [1913 Webster]