Lest

lest

lest/

conjunction

formal

1. with the intention of preventing (something undesirable); to avoid the risk of.

   "he spent whole days in his room, headphones on lest he disturb anyone"

   • (after a clause indicating fear) because of the possibility of something undesirable happening; in case.

     "she sat up late worrying lest he be held up on the way home"

   
   

From: Google.com