Hi Koiyuki,

It's just an old form of adjective 'ない' and still used sometimes to add solemnity in the sentence. The dictionary form is 'なし', 'なき' for when you modify nouns, 'なかろう' is same as 'ないだろう',　'なければ／なけれど' are conjunctional form and 'なかれ' for imperative.

Please don't confuse 'ないで' or 'なくて' with adjective 'ない／なし' as they are suffix to make negative form of verbs but 'ない' or its old form 'なし' is an adjective when they are connected with other adjectives. For example when you say '高く-ない／なし' both '高く' and 'ない' are adjectives but when you say '食べ-ない', '食べ' is verb and '-ない' is a suffix, although their function is similar and both make negative forms. You can say '高くはない' but cannot say *'食べはない'.

In the same way you can make any adjectives into old form such as '高し','おもしろし','悲し','味きなし ' and so on.