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.
Origin: before 1000; Middle English leste, contraction of the lesse the, thi les the;late Old English the lǣste, earlier thȳ lǣs the, literally, whereby less that (thȳ instrumental case of the demonstrative and relative pronoun, lǣsless, the relative particle)
an extraordinary or unusual thing, person, or event; an exceptional example or instance.
a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question.
c.1240, from O.E. 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 other two words contracted into leste.