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 nothing about 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.