This pattern is a double negative that softens a statement. It means you admit something is true, but with hesitation or reservation. For example, if you say 行かなくはない, you're saying "it's not that I won't go"—implying you might go, but you're not fully committed. It often carries a nuance of reluctance or modesty.

Use it when you want to avoid a direct yes or a strong assertion. It's common in spoken Japanese but also appears in writing. The pattern is a bit formal and careful; in casual speech, you might hear なくもない, which is similar but more colloquial. However, なくはない is the standard form for N2.

Note that the pattern is not used with the て-form or the ます-stem. It always attaches to the ない-form. Also, for nouns and な-adjectives, use ないわけではない or ないこともない instead.