What is なく・ない and why is it used?


[This question showed the
following:](https://japanese.stackexchange.com/questions/9546/%E8%A8%80%E3%82%8F%E3%81%AA%E3%81%84%E3%80%80vs%E3%80%80%E8%A8%80%E3%82%8F%E3%81%AA%E3%81%8F%E3%81%AA%E3%81%84-a-negative-
verb-conjugated-as-an-adjective)

言わない (negative)

言わなく・ない (negative+negative)

言わなく・なかった (negative+negative+past)

I've never seen this conjugation before. Why would you have a negative +
negative? Where is that coming from? How does that work exactly?




In addition to the previous answer, often these forms are seen with an
particle in the middle (は or も), and are used followed by for such as が・けど
(examples borrowed/stolen from internet, any translation mistakes my own)

気持ち分からなくはないけど... It's not that I don't understand his feelings, but... (I
**do** understand, but I still don't approve of his actions/won't change my
mind/etc)

まあ、言わなくもないけど… 時と場合によるよ... Well, it's not that I wouldn't say that... it
depends on the situation... (I **would** say that... but maybe not at this
time/in this case)

