Dog watch, in marine or naval terminology, is a watch, a period of work duty or a work shift, between 16:00 and 20:00 (4 pm and 8 pm). This period is split into two, with the 'first' dog watch from 16:00 to 18:00 (4 pm to 6 pm) and the 'last' dog watch from 18:00 to 20:00 (6 pm to 8 pm). Each of these watches is half the length of a standard watch.
The last dog watch is also referred to as the second dog watch in some sources. For instance Jack London in The Sea Wolf, chapter 6: "(rest) I can steal near the end of the second dog watch". The Admiralty Manual of Seamanship from 1913 also refers to the first and second dog watches.
The reason behind this watch's existence is that, in order for the crew to rotate through all the watches, it was necessary to split one of the watches in half, to create an odd number of watches in a ship's day. This allowed the sailors to stand different watches instead of one team being forced to stand the mid-watch every night. The choice of time also allows both watches, if there are only two, to eat an evening meal at about the traditional time.
The Oxford English Dictionary states that the word 'dogwatch' is a direct translation from either German or Dutch of a similar term. It originally referred to the night-watch on ships — that is, the time when (on land) all but the dog were asleep. The name is also said to be derived from Sirius, the "Dog Star", on the claim that Sirius was the first star that can be seen at night. An alternative folk etymology is that the name arose because someone tasked with one of these 'half' watches was said to be 'dodging the watch', taking or standing the 'dodge watch'. This became shortened to 'dog watch'. Another variation is that those sleeping get only 'dog sleep' in this watch. Stephen Maturin of Patrick O'Brian's Aubrey-Maturin series retells the 19th century humourist Theodore Hook's pun that the dog watch is so-named because it is "cur-tailed" ("curtailed", i.e. shortened). Dogging in seaman's terms means securing, such as dogging a hatch. It's called a dog watch because each watch is dogged , or secured prematurely to allow for meal times.