Definition of "lest" in English

conjunction

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

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

2. (after certain expressions denoting fear or apprehension) that (without the negative particle; introduces the reason for an emotion.)

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