The image on the left is suposed to illustrate the phrase "so of a gun". This phrase is used to refer to someone who is a bit of a rough diamond, but a good guy nevertheless-or, interjectionally, to express surprise or disappointment.
In Kydd's day, "wives" of seamen were allowed on board in harbour, and occasionally at sea. As the gangways had to be kept clear, the only place where women in labour could give birth was in the spaces between the guns on the gundeck. Thus a male child born on board was known as "a son of a gun"; collectively they bacame "sons of the sea":
"Begotten in the galley and born under a gun
Every hair a rope yarn, every tooth a marline spike
Every finger a fishhook and his blood right good stockholm tar"
