lest 
(lĕst)
(lĕst)conj.
1. For fear that: tiptoed lest the guard should hear her; anxious lest he become ill.
2. So that someone or something not; in case someone or something might: "Lest anyone forget, adolescence is purgatory" (Jon Pareles).
[Middle English, from Old English (thȳ) lǣs the, (whereby) less that, so that not : lǣs, less; see LESS + the, relative conjunction; see to- in the Appendix of Indo-European roots.]