More to Kendall's explanation above.

A "dog" is an object to hold fast another object and to "dog" something is to therefore hold it in place. Woodworkers have dogs which are metal staples or springy bars and levers to keep the piece from moving as you work on it. I don't know if there is a dog specially designed for nautical use but the most common type was a large forged iron staple that could be hammered on and easily pried off.