double negative
noun
plural double negatives
: a now nonstandard syntactic construction containing two negatives (see negative entry 2 sense 3a) and having a negative meaning
"I didn't hear nothing" is a double negative.
Note: Double negatives are nonstandard in Standard American English. When two negatives occur together, the sentence is often criticized as an ungrammatical way to express a positive meaning, with the negatives interpreted as canceling each other out. But such constructions are common and grammatical in other varieties of English, including African American English, Southern, and Appalachian English. In these language varieties, two negatives amplify one another, as in, "She ain't never lost a race," and "We didn't have no dog then."
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Merriam-Webster unabridged



