Lest

[lest] /lɛst/

conjunction
1.
for fear that; so that (one) should not (used negatively to introduce a clause expressive of an action or occurrence requiring caution):
He kept his notes by his side lest faulty memory lead him astray.
2.
that (used after words expressing fear, danger, etc.):
There was danger lest the plan become known.
/lɛst/
conjunction (subordinating; takes should or a subjunctive verb)
1.
so as to prevent any possibility that: he fled the country lest he be captured and imprisoned
2.
(after verbs or phrases expressing fear, worry, anxiety, etc) for fear that; in case: he was alarmed lest she should find out
conj.

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