lest
conj. C2 Proficiency US //ˈɫɛst// UK //lˈɛst// lest
conj. used to say that you do something to prevent a bad result. It means 'so that... not'.
conj. a conjunction introducing a clause of purpose, specifically to avert a negative outcome. Formal and literary; often triggers the subjunctive mood in traditional usage.
He whispered lest she wake up.
The team worked quietly lest they disturb the sleeping baby.
She kept her voice low lest the sudden outburst reveal her true feelings to the room.
Usage
The conjunction is followed by a clause. In formal writing, the verb in that clause often takes the subjunctive mood (e.g. 'lest he forget') or uses 'should'.