Auxiliary verb
(does) not ..., and; (does) not ..., so; not (doing); without (doing)
after the -nai stem of a verb and for サ変 as せ～ず (all senses); continuative form of ぬsee ぬsee ずに
彼は健康がすぐれず、辞職せざるをえなかった。
He was compelled to resign on account of ill health.