Two negatives make a positive in modern English grammar, causing some distress to confused sign-makers.
Yet is was not always thus. Before the 17th century you could play fast and loose with your negatives, piling them on top of each other in a cumulative no-no of denial.
‘Ther nas no man nowher so vertuous…’ (Chaucer, ‘The Friar’s Tale’)
And of course, there’s Milton. The great sentence-twister whose use of syntax makes a Balinese dancer look clunky and whose double negatives conjure a whole universe of rebellion, regret, sin and wrong-thinking, exact and to order.
Consider the difference between ‘aware’ and ‘not unaware’. Essentially they should mean the same thing. But we’re back to Milton. ‘Aware’ is pure, simple, only just awake. ‘Not unaware’ is aged, circumspect, mistrusting, shadow-lurking and afraid. There is a time and place for all. Sometimes a skilful double negative will sound a lexical minor key with a subtlety inaccessible to other techniques.
Colloquial (and grammatically incorrect) double negative use often hearkens back to the cumulative effect of centuries ago. ‘I don’t know nothing.’ There’s nothing, and there’s the not knowing anything, all heaped up on top of each other. We can’t possibly translate this as knowing something, and ignore the rich emptiness it brings to the table. After all, it’s not unusual to find that we are surprisingly close to our roots, even unknowingly.