A dogsbody, or less commonly dog robber in the Royal Navy, is a junior officer, or more generally someone who does drudge work. A rough American equivalent would be a "gofer," "grunt" or "lackey".
The Royal Navy used dried peas and eggs boiled in a bag (pease pudding) as one of their staple foods circa the early 19th century. Sailors nicknamed this item "dog's body". In the early 20th century, junior officers and midshipmen who performed jobs that more senior officers did not want to do began to be called "dogsbodies". The term became more common in non-naval usage ca. 1930, referring to people who were stuck with rough work.
The term dogsbody has not always been derogatory, with a number of people deliberately using it as their callsign or handle. The most famous of these is probably Douglas Bader, who was an RAF fighter pilot during the Second World War.
Dogsbody is a 1975 children's novel by Diana Wynne Jones.
Sirius, guardian luminary of the Dog Star, has been convicted of murdering a fellow "luminary" and of losing the Zoi, an extremely powerful cosmic tool that he has allowed to fall to a minor planet. He is sentenced to spend one lifetime in the form of dog, an animal native to this planet: if he can recover the Zoi within that dog's lifetime, he will be allowed to return to his former status as Sirius. If he does not, he will simply die at the end of his dog's life. Sirius accepts the sentence and is born into a litter of puppies.
Discovered as mongrels, the puppies are thrown into the river in a sack. This is just the beginning of Sirius's problems. Although adopted by the loving Kathleen, he learns that she is "low dog" everywhere because she is Irish. Struggling with his limitations as a dog and his perceptions as a star, coping with the bigotry in the household, trying simply to get out on the street so he can begin his search for the Zoi, Sirius is battered by one setback after another. How he - with help from Kathleen, the luminary Sol (our sun), and his own quick wits - uses his canine and stellar wisdom to track the lost cosmic tool, is an intricate and intriguing tale.