A very long time ago. One explanation suggests that the expression might have become popular in the 1920s when many schoolboys studied Greek and had dogs named Hector after the Homeric hero. Another possibility is also rooted in classical studies: according to the playwright Euripides, Hector's mother, Hecuba, was turned into a dog for murdering the killer of her older son; therefore, Hector was the son of a dog, which made him a pup. In any event, the phrase is now obsolete.