/lest/ , conj.
1. for fear that; so that (one) should not (used negatively to introduce a clause expressive of an action or occurrence requiring caution): He kept his notes by his side lest faulty memory lead him astray.
2. that (used after words expressing fear, danger, etc.): There was danger lest the plan become known.
[ bef. 1000; ME leste, contr. of the lesse the, thi les the; late OE the laeste, earlier thy laes the, lit., whereby less that ( thy instrumental case of the demonstrative and relative pronoun, laes LESS, the relative particle) ]