This phrase means, “a very long time ago.”
The expression became popular in the 1920s when many schoolboys studied Greek and named their dogs Hector after the Homeric hero. The phrase is now obsolete, though there are similar ones still in use.
For example, in the US military, it’s apparently common to refer to a period in the distant past as one when Christ was a corporal. Those associated with the RAF in World War Two will know that a time in the distant past was one when Pontius was a pilot.
A further variation on the theme is “when Pluto was a pup.” As the first example in print appears to be from a newspaper advertisement dated 1947, it may have been derived from Disney’s dog, who was named in 1931 after the then newly discovered (and now demoted) planet.
The phrase began to appear in North American newspapers circa 1906 and became a catchphrase that spread through the English-speaking world. By this time, “pup” was also well established as a somewhat dismissive name for a young person, particularly an inexperienced beginner.
Eric Partridge, in A Dictionary of Catch Phrases American and British, says it goes back to about 1920. Hector used to be a common name for a large dog.
The Oxford English Dictionary labels the phrase as an American colloquialism. “Hector may refer to the Trojan hero who lived around 1200 BC and whose mother Hecuba was, according to Euripides, turned into a dog (hence Hector could be regarded as her pup); it may also reflect the popularity at various times of Hector as a dog’s name.”
The OED’s earliest citation for the expression is from an 1895 issue of the Washington Post: “[They] have been scrapping with each other in this neighborhood ‘since long before Hector was a pup.’ ”
Being a cat person, perhaps I could start a new trend. How about:
— since Cleopatra was a kitten
— since Nefertiti was an itty-bitty kitty
No? Oh, well.