lest

conj

1. (formal, literary)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.”

   “I thought to haue told thee of it, but I fear'd / Leaſt I might anger thee.”

   “And then Robert Loo came out swiftly with the half-filled jar lest more be said.”

   “Stay free from petty jealousies / Live by no man's code / And hold your judgment for yourself / Lest you wind up on this road”

2. (formal, literary)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.”

   “That you and I should be in the same house together and not able to speak to each other is in itself a misery, but this is terribly enhanced by the dread lest this state of things should be made to continue.”

   “[M]y ward, or rather my adopted son Leo Vincey and myself have recently passed through a real African adventure, of a nature so much more marvellous than the one which you describe, that to tell the truth I am almost ashamed to submit it to you lest you should disbelieve my tale.”

   “Mr. Cooke at once began a tirade against the residents of Asquith for permitting a sandy and generally disgraceful condition of the roads. So roundly did he vituperate the inn management in particular, and with such a loud flow of words, that I trembled lest he should be heard on the veranda.”