A correspondent writes to say she had read an interview in Rolling Stone with the cast of AMC’s Mad Men, where one of the actors said they had to be very careful with their pronunciation. 'There was no ‘gonna’ or ‘shoulda’ back then [in the 1960's].' Could this be true?
It certainly couldn't. It's easy enough to check the point. These two forms actually get separate entries in the OED, and there we find gonna with a first recorded use in 1913 and shoulda with a first recorded use in 1933. (Also woulda, 1913, coulda 1925.)
Both usages are undoubtedly much older. Joseph Wright's English Dialect Dictionary has a separate section under go giving examples spelled ganna, gauna, gaunna, and ginnie. The earliest is 1806. And under have there's an example of should ha' from 1899.
Why stop there? In the 1602 Quarto edition of Merry Wives of Windsor we find Nym saying 'I should ha borne the humor Letter to her', and there are several similar examples in the literature of the period.
When it comes to linguistic mythology, it's seems it's a mad Mad world.