It must be remembered that two negatives in the English language destroy each other and are equivalent to an affirmative.
Thus “I don’t know nothingabout it” is intended to convey, that I am ignorant of the matter under consideration, but it defeats its own purpose, inasmuch as the use of nothing implies that I know something about it. The sentence should read—”I don’t know anything about it.”
Often we hear such expressions as “He was not asked to give no opinion,” expressing the very opposite of what is intended. This sentence implies that he was asked to give his opinion.
The double negative, therefore, should be carefully avoided, for it is insidious and is liable to slip in and the writer remain unconscious of its presence until the eye of the critic detects it.