Online Merriam-Websters dictionary....

One entry found for lest.

Main Entry: lest
Pronunciation: 'lest
Function: conjunction
Etymology: Middle English les the, leste, from Old English thy l[AE]s the, from thy (instrumental of thæt that) + l[AE]s + the, relative particle
Date: before 12th century
: for fear that -- used after an expression denoting fear or apprehension - worried lest she should be late - hesitant to speak out lest he be fired