Source : Webster's Revised Unabridged Dictionary (1913)

Sea dog \Sea" dog`\
   1. (Zo["o]l.) The dogfish.
      (b) The common seal.

   2. An old sailor; a salt. [Colloq.]

Source : WordNet®

sea dog
     n : a man who serves as a sailor [syn: {mariner}, {seaman}, {tar},
          {Jack-tar}, {Jack}, {old salt}, {seafarer}, {gob}]