せず - What does it come from?

Question

気にするまいと思っても、気にせずにはいられない。

In above sentence, I guess せず means しない, but what is the original form, and how does it become せず?

Answer 1

The -ず form of a verb means "without doing" and is roughly equivalent to ないで. It's not a contraction of anything and is a verb form on its own.

The -ず form is formed by taking the -ない form of a verb and replacing the -ない with -ず. For the irregular verbs, する becomes せず and [来]{く}る becomes [来]{こ}ず.

Answer 2

what is the original form

The verb する.

and how does it become せず?

ず is an auxiliary verb ([助動詞]{じょどうし}) which attaches to the [未然形]{みぜんけい} of the verb. せ is one 未然形 of する and is the one which ず attaches to.