Webster's Revised Unabridged Dictionary · 1913

Lest

Lest (lest) , intransitive verb

To listen. [Obsolete] — Chaucer. Spenser

Lest , noun

[See List to choose.]

Lust; desire; pleasure. [Obsolete] — Chaucer

Lest , adjective

Last; least. [Obsolete] — Chaucer

Lest , conjunction

[Old English leste, from Anglo-Saxon eȳ las eē the less that, where is the instrumental case of the definite article, and is an indeclinable relative particle, that, who, which. See The, Less, a.]

1.
For fear that; that... not; in order that... not.
Love not sleep, lest thou come to poverty. — Bible (KJV) - Proverb xx. 13
Wherefore let him that thinketh he standeth take heed lest he fall. — 1 Cor. x. 12
2.
That (without the negative particle); -- after certain expressions denoting fear or apprehension.
I feared Lest I might anger thee. — Shakespeare