Learn English – Are “not uncommon” and similar phrases double negatives? Should their use be avoided

Double negatives can be perfectly fine in English.

• If their sum is supposed to be negative, double negatives are very informal or slang in modern English. This usage is frowned upon by many people even if used in speech, unless ironically.

1.) I don't see nothing. (= I don't see anything.)

• If their sum is supposed to be positive, it is generally acceptable in all registers:

2.) I suppose that is not impossible. However, it seems far fetched.

Here the double negative expresses a weak positive, a very common construction.

3.) Not bad, not bad at all! You have just saved her life, young man.

This is a figure of speech called litotes: the double negative (if that's what it is) is used to express a strong positive. Sometimes any double negative with a positive meaning is considered a litotes, including the unremarkable example 2 above. Other people restrict the term to those negations that express a strong positive through an apparently weak positive, in a mildly ironical manner, as in this example (3).

4.) Never a day goes by that I do not miss her. (= I always miss her.)

This is the rhetorical double negative, often considered a form of litotes. It expresses a strong positive, though without irony.

5.) Well, I didn't not enjoy it, but...

Here the word not is used twice, once in contracted form (don't) and once in full, to express a weak positive. Double not is a special case: it is felt to be even more redundant than other double negatives and sounds rather colloquial. The majority will probably use this in speech and informal writing, where it is perfectly acceptable, but not elsewhere.

The boundary between negations and other kinds of words is by no means sharp. The prefixes un- and im-/in- are usually considered negatives, as are hardly and many others; bad is sometimes considered a negative word, sometimes not, etc.

This is, I think, an instance of ‘baby-talk’ (what linguists call child-directed speech), which is a special dialect adopted by adults in speaking to small children. Although medica is undoubtedly correct in asserting that children employ such terms as “pretty please”, “boo-boo”, “yucky” with each other, I believe they learn them in the first instance from adults.

Some elements of baby-talk—simplified syntax, a deliberately narrowed lexicon, morphological adjustments to mark word and phrase boundaries more clearly—are believed to be of value in providing a child more intelligible models for imitation. But there is a large range of ‘styles’ within this dialect, and many people adopt distinctive (mis)pronunciations and words and phrases which they suppose are characteristic of child speech, as if the child's progressive acquisition of language were not an awe-inspiring and heroic achievement but a species of imbecility to be catered to. At one time it was common in novels to represent the speech of children as baby talk—very like the demeaning representations of African-American or rural or foreign speech which once passed for humor on the stage and in literature.

Baby-talk is also used to pets, and with markedly ironic intent in ordinary speech between adults. And some children, having found that baby talk is regarded as ‘cute’ and endearing, carry some of it terms into adolescence and even adulthood. My impression is that this is less common than it was fifty years ago; I hope I am right.