In the innocent days of the 1920s-30s, when Richmal Crompton published all those magazine stories of William Brown ['Just William'] & his friends The Outlaws, all reissued in book form until after her death in 1969, the word "couldn't" as pronounced by the 4 boys was always rendered as "cun't". I have always been amazed that nobody ever drew attention to this & alerted the publishers Newnes as to the unfortunate ambiguity.

In the S Wales valleys the problem continues, 'cun' = I am able, as in "I cun do that". The misspronunciation doesn't appear to carry over into the negative though.