This word has a perfectly delicious definition:

Used as a negative particle of intention or purpose, introducing a clause expressive of something to be prevented or guarded against;

It comes from Old English by a fairly common route of dropping sounds and syllables until it has become one clear syllable.  Original meaning: “whereby less”

• 06.088 and fear lest he fall off that narrow place
• 08.081 lest they should discover him.
• 11.033 Quickly, trembling lest the chance should fade,
• 16.005 and I fear lest there be battle

The use of “lest” peaked in the second half of the seventeenth century and dropped off after 1850, thanks Google Ngram Viewer.  While OED does not call it archaic, it has a high sound to me.

“lest, conj.” OED Online. Oxford University Press, March 2015. Web. 11 May 2015.