Double negatives in court via @CrimeLineLaw pic.twitter.com/b8xSrlny2V
— Roddy Mansfield (@roddymansfield) January 24, 2015
It's the old chestnut that a double negative actually makes a positive, and if you say the non-standard phrase 'I didn't do nothing' you are in fact saying that you did something. It's used to try to shame or humiliate people into using the Standard English single negative: 'I didn't do anything'.
This is quite silly. Generally, linguists point out that lots and lots of other languages have double negatives as the standard form, and so it's ridiculous to suggest that it's somehow illogical. Italian is yer go-to example here, and for some reason it's always about telephoning: 'non ha telefonato nessuno', or 's/he hasn't telephoned nobody'. Jack Chambers has pointed out that as most non-standard English varieties have double negation (more technically called negative concord), perhaps it is in some way more 'natural' than the artificial standard of single negation that is imposed on us.
But even more than this, it's not even true that a double negative will be interpreted as a positive. It can be, if you give it the right intonation. But it's a very specific intonation with a pretty strong emphasis on the 'nothing'. Without that, there's simply no way that it can possibly mean 'I did something'. Anyone at all would interpret it was meaning what it's meant to mean. They might be a pedant about it and pretend not to understand, but they definitely would. And even in a criminal trial, where testimony has to be unambiguous, I don't think that they would try to hang the crime on the guy for using this syntactic construction.