Double negatives and usage

In standard English, when we use negative words such as nobody, nowhere, never or nothing, we do not commonly use a negative verb:

He had nothing interesting to tell us.

Not: He hadn’t nothing interesting…

It was 10 am but there was nobody in the office.

Not: … but there wasn’t nobody in the office.

However, we hear double and triple negatives spoken in some regional dialects of English. This is common when people from the same region are speaking with one another. Double negatives like this are not acceptable in formal situations or in writing.

Compare

non-standard dialect	standard English
We couldn’t never work with nobody like that.	We couldn’t ever work with anybody like that.
He never says nothing interesting to no one.	He never says anything interesting to anyone.

Double negation with adjectives and adverbs (not unexpected)

However, we can use not + an adjective or adverb with a negative prefix (e.g. un-, in-) as a way of softening or downtoning the meaning of the adjective. The meaning becomes affirmative, but the double negation shows that the writer/speaker is cautious about it. This is most common in formal writing:

This year’s rise in inflation to 3% was not unexpected. (This year’s rise in inflation to 3% was expected to some extent.)

The crisis has been attributed, not unreasonably, to the Prime Minister’s weakness.

A:
The repair cost fifty pounds.

B:
Oh well, that’snot unreasonable.