double negative

In grammar, a construction that uses two negatives instead of one to convey a negative meaning (for example, ‘I don't want no trouble’). This is not Standard English but is common in some dialects.

Standard English accepts two types of double-negative construction: with an adjective that contains a negation (‘it is not unusual’); and in sentences where the two negatives reinforce each other (‘He would never have done it, not even if his life depended on it’). Sentences of the type ‘I wouldn't be surprised if that wasn't the messenger at the door now’ are considered informal.