Noun
A pirate.
A sailor accustomed to the sea.
The old seadog knew the storm was coming long before the rest of the crew were aware of it.
(heraldry) A charge resembling the talbot but with scales, webbed feet and a broad tail.
A dogfish.
(uncommon) An omen of bad luck.
(uncommon) A white-topped wave in an estuary.
(uncommon) A fogbow, as seen by mariners.
Source: en.wiktionary.org