Emily Dickinson Lexicon

LEST, part. [from the Sax. leas, Goth. laus; louse, separate. In Saxon it was preceded by the, the leas, that less, that not, ne forte. Hence it denotes a loosing or separation, and hence it comes to express prevention.]

That not; for fear that. Ye shall not eat of it, neither shall ye touch it, lest ye die. – Gen. iii. The phrase may be thus explained. Ye shall not touch it; that separated or dismissed, ye die. That here refers to the preceding command or sentence; that being removed or not observed, the fact being not so, ye will die. Sin no more, lest a worse thing come to thee. John v. Sin no more; that fact not taking place, a worse thing will happen to thee.