lest /lest/ BrE AmE conjunction literary
[ Language: Old English ; Origin: thy læs the 'by which less that' ]
1 . in order to make sure that something will not happen:
She turned away from the window lest anyone see them.
2 . used to show that someone is afraid or worried that a particular thing might happen
worried/concerned/anxious etc lest ...
He paused, afraid lest he say too much.
She worried lest he should tell someone what had happened.