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 eȳ is the instrumental case of the definite article, and eē 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