Double negation of an i-adjective

Question

Does it exist as such, the double negation of an i-adjective, e.g.: おいしくなくない? I was told that I should use おいしくなく　は　ない instead. Is that right? Is it a rule? Would it depend on context which was a question like: Are the meals at that restaurant good? Or is it just the style of the person I talked to?

Of course, I could also use e.g. まずくない instead, but this is not the question here.

Answer 1

The double negation of i-adjectives not only exists, but it is quite commonly used among us native speakers when expressing opinions indirectly.

Take 「おいしい」 ("tasty") for example, by far the most common double-negative form would be:

「おいしくなくはない」

which means:

"(the food) is okay/passable if not great"

That sounds fairly indirect, doesn't it? The direct phrase would simply be 「まずい」 ("bad-tasting") or 「おいしくない」, which would often be considered too direct for the Japanese taste (pun intended).

We also say:

「おいしくなくもない」

which means practically the same thing as 「おいしくなくはない」.

I do not think that careful speakers would use 「おいしくなくない」 with no particle between the 「なく」 and 「ない」 to mean the same as the two above. I would, however, not be surprised if I heard it in careless hurried speech.

Answer 2

I think you may occasionally hear some ギャル say これ美味しくなくない？ when, for example, going to a highly reviewed restaurant, being somewhat disappointed by the food, then asking a friend for affirmation.

In normal speech, however, you will quite commonly hear people say double negatives with a particle in between.

美味しくなくはない。
It doesn't taste bad.

忙しくなくはない。
I wouldn't go so far as to say "not busy".

Answer 3

From a grammatical point of view you can make double or triple or whatever-iple negations, though it's not something you'd throw in a normal conversation. As an example from Ace Attorney:

-そんなことないよ。
-そんなことなくないッ！
-そんなことなくなくない！
-そんなことなくなくなくなくなく...

With exception of comic effect I hardly can imagine situation where you may need this.