やらん

Etymology 1
Contraction of phrase にやあらむ, consisting of  +  +  +. Literal meaning is roughly “isn't it so?”

→ →

The yaramu form is already seen in the Man'yōshū, dated to around 760.

Phrase

 * 1)  “isn't it?”
 * When used at the end of a sentence. Replaced in modern standard Japanese by in casual informal speech, and  in more generally polite speech.
 * 1)  expresses uncertainty or indefiniteness
 * When used mid-sentence following a noun. Generally preceded by adverbial particle.

Etymology 2
Contraction of verb form, from verb.

→

Verb

 * 1)  not do
 * See for details.