lest

For fear that; that not; in order to prevent something from happening; in case.formalliterary

• He won't go outside, lest he be eaten by those ravenous eagles.
• I brought my notes lest faulty memory lead me astray.

that (without the negative particle; introduces the reason for an emotion.)formalliterary

• There was danger/alarm lest the plan become known.
• I am afraid lest I revealed too much.