Well "cod" just means "imitation" so I'd guess it means he didn't speak Yiddish at all just made sounds that sounded vaguely Yiddish in tone and sprinkled in the odd Yiddish word.
It's a bit like adopting a vaguely French accent and saying "oui" and "non" every now and then.
"No man but a blockhead ever wrote except for money." Samuel Johnson.