Source : Webster's Revised Unabridged Dictionary (1913)

Lest \Lest\, n. [See List to choose.]
   Lust; desire; pleasure. [Obs.] --Chaucer.

Lest \Lest\, a.
   Last; least. [Obs.] --Chaucer.

Lest \Lest\, conj. [OE. leste, fr. AS. [eth][=y] l[=ae]s
   [eth][=e] the less that, where [eth][=y] is the instrumental
   case of the definite article, and [eth][=e] 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. --Prov.
                                                  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.     --Shak.

Lest \Lest\ (l[e^]st), v. i.
   To listen. [Obs.] --Chaucer. Spenser.

Source : WordNet®

lest
     adv 1: for fear that; "she tiptoed lest her mother should hear her"
     2: that (after verbs explicitly expressing fear etc.); "he
        worried lest he should be late"