Learn English – What does “by and by” mean

I suspect that there are several word-plays going on here. In American myth and folklore, the full moon of each month is given a name, such as Wolf Moon, Storm Moon, Harvest Moon, Hunter's Moon, etc. A blue moon is a rare occurrence that happens about once every 2-3 years when a single season (three months) experiences four full moons; the third full moon in a season with four full moons is called the Blue Moon. There is no full moon specifically called the Killing Moon.

An Old Testament poem tells us that there is a time for everything under the sun; a time to give, a time to take, a time to heal, and a time to kill, etc. Calling something a killing time or killing moon would mean that somebody's or something's death is imminent and unavoidable.

I've looked up the entry in several slang dictionaries and there seem to be at least two meanings.

• When applied to children, that of silly and poorly educated with a whiff of mischievousness.
• When applied to adults, that of foolish and/or ridiculous.

Below are my sources, cited in extenso.

Oxford Dictionary of Slang (1998)

’erbert n British
A foolish person, a cheeky, unwashed child. For many years, in London working-class slang, Herbert or ’Erbert was used to refer to any otherwise unnamed man or boy. Gradually, probably by being used in phrases such as ‘silly ’erbert’, it came to have the more pejorative sense. There probably never was an eponymous Herbert; it was merely a common working-class name from the Edwardian era.

The Concise New Partridge Dictionary of Slang and Unconventional English (2008)

herbert noun
 1. a mischievous child or youth. Quite often heard as ‘little herbert’ UK, 1999.
 2 a harmless youth; a ridiculous man. An extension of the previous sense UK, 1960.
 3 a man in a specified field of endeavour UK, 1956

John Ayto Oxford Dictionary of Slang (1998)

Herbert (1960) British;
applied to a foolish or ridiculous man ; arbitrary use of the male forename
T. Barling: A dozen baby-brained herberts looking to face me off just to say they squared up to Kosher Kramer before the cobbles came up a bit smartish. (1986)