Learner's definition of LEST
formal + literary
for fear that
— used to say that you do not want something to happen
— used when you are saying something in order to prevent something from happening
And lest you think I'm joking, let me assure you that everything I've said is true.
She's a talented singer and, lest we forget, a fine musician as well. [=we should not forget that she is also a fine musician]