~ku wa nai ～くはない

In Japanese, ~ku wa nai ～くはない is the adverbial form of an i-adjective, plus the wa は particle, plus the negative hojo-keiyoushi 補助形容詞 nai ない. Or it might be kuwanai 食わない, "won't eat."

Basically, ~ku wa nai is used to affirm something "is not" a given adjective.

• warui?
  悪い？
  Is [it] bad?
• waruku wa nai kedo..
  悪くはないけど･･･
  Bad, [it] is not but...
  • It's something else. (this is a contrastive wa は.)

See ~ku wa ～くは for details about the grammar.

Manga: Mairimashita! Iruma-kun 魔入りました！入間くん (Chapter 12, 入間の“夢”)

• Context: a human boy ends up in the demon world.
• ningen-kai ni modoritaku wa nai no desu ka?
  人間界に戻りたくはないのですか？
  Don't [you] want to return to the demon world?
  • modoritai
    戻りたい
    Want to return.
  • modoritakunai
    戻りたくない
    Not want to return.