ないことはない is an expression that utilizes either the 助動詞（じょどうし） (auxiliary verb) ない, or the
い-Adjective ない, depending on what type of word it is used with. This structure may literally be translated as something similar to ‘not (A) is not all there is’. However, the common translations are ‘(A) is not impossible’, or ‘it doesn’t mean that it isn’t (A)’.
This expression may be used with the ない form of any verb,
い-Adjective, or
な-Adjective. Let’s have a look at a few examples before exploring the meaning further.
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映画（えいが）は見（み）ないことはないが、詳（くわ）しくもない。
It’s not that I don't watch movies, but I am just not knowledgeable about them at all.
ここは危（あぶ）ないことはないけど、気（き）をつけた方（ほう）がいい。
It’s not that this place isn't safe, but you should be careful.
元気（げんき）ではないことはないけど、すごく元気（げんき）なわけでもない。
It doesn't mean that I am not healthy, but it is not the case that I am very fit.
By itself, ことはない highlights something that is unneeded, or never happens. This is partly due to the 副助詞（ふくじょし） (adverbial particle) は expressing something as being ‘complete’. In this way, ‘there is no situation in which (A)’, is quite an accurate translation of ことはない. By adding ない to the (A) verb, all
ないことはない is doing is simply changing this nuance to ‘there is -not- no situation in which (A)’, a double negative expressing that (A) is actually possible.
ないことはない often appears with verbs in the potential form. This usually implies a lack of effort, or feelings of half heartedness toward the completion of (A). It is similar to ‘it’s not like I can’t (A)’ in English.
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納豆（なっとう）は食（た）べられないことはないけど、自分（じぶん）から買（か）って食（た）べようとは思（おも）わない。
It's not that I don't like natto, it's just that I don't think I will buy it myself to eat.
歩（ある）いていけないことはないけど、歩（ある）いたら6時間（じかん）ぐらいかかるよ。
It's not that you can't walk there, but if you did, it would take about six hours.