あらず

From Old Japanese. As a basic verb form, attested in ancient sources such as the Man'yōshū of 759.

The interjection senses appear later in the historical record, with the no, wrong sense attested from around the late 800s in the Kokin Wakashū as a shortening of the phrase さにはあらず (sa ni wa arazu, literally “it's not like that”),[1][2] and the never mind sense attested from roughly the late 900s as recorded in The Pillow Book.[1][2]

Derived from the copula あり (ari) as the regular negative form, from the 未然形 (mizenkei, “irrealis conjugation”) of ara- + negative auxiliary suffix ず (zu).

あらず • (arazu) 

1. 非ず, 有らず, 在らず: [from 700s] (Classical Japanese or literary) negative of あり
   • 1888, 森鷗外『獨逸日記』

     味(あじ)は甚(はなは)だ美(び)ならず。然(しか)れども必(かなら)ずしも厭(えん)嫌(けん)す可(べ)きに非(あら)ず。

     Aji wa hanahada bi narazu. Shikaredomo kanarazu shimo enken subeki ni arazu.

     The flavor is not very good. However, it is not necessarily [something] that should be detested.

   • 1922, 富井政章『民法原論』

     負(ふ)担(たん)と条(でう)件(けん)とを識(しき)別(べつ)することは実(じつ)際(さい)困(こん)難(なん)なる場(ば)合(あひ)なきに非(あら)ず。

     Futan to jōken to o shikibetsu suru koto wa jissai konnan naru baai naki ni arazu.

     Distinguishing responsibility and conditions is, in reality, not without difficult cases.

• The equative 'X is not Y' sense, being the negative form of にあり, always follows the copular particle に (ni), sometimes in combination with contrastive topic particle は (ha). The existential sense may stand isolated.

• ならず (narazu) (contraction of に (ni) + あらず (arazu))

• ではない (de wa nai)

あらず • (arazu) 

1. 非ず: [from late 800s] (archaic, possibly obsolete) no, wrong
2. 非ず: [from late 900s] (archaic, possibly obsolete) never mind