This phrase first began to appear in North American newspapers around 1906 and became a catchphrase in the 1920s, especially among flappers. It was an extension of an earlier idiom – “as dead as Hector” — which was widely used in the 1860s.
The reference is to Hector, the son of King Priam of Troy and his second wife Hecuba — a symbol of the consummate warrior — and one of the chief participants in the tale of the siege of Troy by the Greeks in Homer’s epic The Iliad. King Priam, as we all know, was killed in single combat by the Greek champion Achilles.
Hecuba was responsible for the murder of Polyxena, who was the murderer of Hecuba’s older son, Polydorus. The gods turned Hecuba into a dog as punishment for taking Polyxena’s life which, literally speaking, made Hector his mother’s pup.
What’s more, by the early twentieth century, “pup” was well established as a mildly dismissive comment that referred to a young person who was particularly inexperienced in the ways of the world.