lest

Etymology

c. 1200, contracted from Middle English les te (“less that”), from Old English þȳ lǣs þe (“whereby less that”), from þȳ (instrumental case of demonstrative article þæt “that”) + lǣs (“less”) + þe (“that,” relative particle). The þȳ was dropped and the remaining two words contracted into leste.

conj

1. For fear that; that not; in order to prevent something from happening; in case.
   He won't go outside, lest he be eaten by those ravenous eagles.
   And then Robert Loo came out swiftly with the half-filled jar lest more be said. 1959, Anthony Burgess, Beds in the East (The Malayan Trilogy), published 1972, page 565
   Stay free from petty jealousies / Live by no man's code / And hold your judgment for yourself / Lest you wind up on this road 1967, “I Am a Lonesome Hobo”, in Bob Dylan (music), John Wesley Harding
   Lest any astrologer reading this result get cocky, Dr Cajochen does not believe that what he has found is directly influenced by the Moon through, say, some tidal effect. What he thinks he has discovered is an additional hand on the body’s clock-face. 27 July 2013, “Lunacy?”, in The Economist, volume 408, number 8846
2. (after certain expressions denoting fear or apprehension) that (without the negative particle; introduces the reason for an emotion.)
   There was danger lest the plan become known.
   I am afraid lest I revealed too much.