sea-dog

noun

1. a sailor, especially an old or experienced one.
2. harbor seal.
3. a dogfish.
4. a pirate or privateer.

noun

1. an experienced or old sailor

n.1590s, “harbor seal,” from sea + dog (n.). Also “pirate” (1650s). Meaning “old seaman, sailor who has been long afloat” is attested from 1840.