A classical/literary negative auxiliary attached to a verb's irrealis (未然形) stem to mean "not~". Often appears in written or set expressions (e.g. 言わざるを得ない, 知らざる者).
彼の行動は問題だと言わざるを得ない。
I have to say his behavior is problematic.
知らざる者には想像しがたい話だ。
It's a story hard to imagine for those who don't know.
古い書物には『なすべきことをせざるは恥』のような表現が見られる。
In old books you can find expressions like 'to not do what one ought is shameful.'