lest
/lest/conjunction
1- formal with the intention of preventing (something undesirable); to avoid the risk of〈正式〉以免（坏事的发生）; 免得; 唯恐：
he spent whole days in his room, headphones on lest he disturb anyone.
他整天整天待在屋里, 头上戴着耳机, 以免打扰别人。
1.1
- (after a clause indicating fear) because of the possibility of something undesirable happening; in case用在表示害怕的从句后面怕, 担心：
she sat up late worrying lest he be murdered on the way home.
她满心焦虑守候到很晚, 怕他在回来的路上会被谋杀。
USAGE
There are very few contexts in English where the subjunctive mood is， strictly speaking， required: lest remains one of them. Thus the standard use is she was worrying lest he be attacked （not lest he was ） or she is using headphones lest she disturb anyone （not lest she disturbs anyone）. See also SUBJUNCTIVE.
词源
Old English ty lǣs the 'whereby less that'， later the læste.