Lest means in case something unwanted should happen. Lest we forget, lest anybody think otherwise, lest it be lost.
It is formal and takes the subjunctive: lest he be late, lest it fall — which is one of the last places English uses that form. Lest we forget is the phrase on war memorials, from Kipling. In American usage lest is also followed by should. Note that lest is one of the few English conjunctions with a built-in negative: it already means in order that not, so lest not is a mistake.
1prepositionlestIn case something unwanted should happen.
“She wrote the date on her hand lest she forget it.”
lest is a conjunction of precaution.
Old English thy laes the meant whereby less that. The whole phrase collapsed into one word, which is why lest has a negative meaning built into it and no visible negative in it.
Old English
þȳ lǣs þe — whereby less that
Middle English
les te — less that
English, today
lest
Built from
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.
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire