Learn English – New (slang?) meaning of bass

Looking in COCA, how come is very common in American English. It's used to mean why, in questions, and occasionally in statements:

How come the reporters aren't asking that?
And that's how come this song "This Land Is Your Land" became known throughout America.

If we break down the places it appears, a pattern emerges: how come is almost never used where formal writing is demanded, as in academic writing, but is frequently used where informality is okay, as in fiction dialogue and speech:

           TOTAL    SPOKEN  FICTION  MAGAZINE  NEWSPAPER   ACADEMIC
HOW COME   2689     882     1318     283       164         42  

In American English, at least, how come? is informal, but probably not considered slang.

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)