A somewhat formal/old-fashioned phrase meaning one has no choice but to use or do something; commonly rendered in modern Japanese as 使わざるを得ない / 使わざるをえない.
古いソフトしか対応していないので、互換モードを使わざるを得ず作業を続けた。
Because only old software was supported, I had no choice but to use compatibility mode and continued working.
予算が限られているため、中古の機材を使わざるを得ず性能に我慢した。
With a limited budget, I was forced to use secondhand equipment and tolerate its performance.
交通機関の乱れで電車を使わざるを得ず、遅刻してしまった。
Due to disruptions in transportation, I had no choice but to take the train and ended up being late.