A “fogdog” is a nautical term that means a bright or clear spot in a bank of fog. For sailors caught in thick fog, a fogdog can sometimes be the only means of finding the shore.

According to some 19th-century nautical manuals, fogdogs are thought to indicate that the fog will lift soon.

According to the Oxford English Dictionary, the earliest known use of the fogdog occurred in 1831, in the Athenaeum, a London journal of literature, science and fine arts.