Lest

google

ref

Old English thȳ lǣs the ‘whereby less that’, later the læste .

wiktionary

ref

c. 1200, contracted from Middle Englishles te(“less that”), from Old English þy læs þe(“whereby less that”), from  þy (instrumental case of demonstrative article þæt “that”) +  læs(“less”) +  þe(“the”). The þy was dropped and the remaining two words contracted into leste. [1]

etymonline

ref

lest (conj.)

c. 1200, "that not," especially "for fear that" [OED calls it a negative particle of intention], from a contraction of the Old English phrase þy læs þe "the less that," from þy, instrumental case of demonstrative article þæt "that" + læs (see less) + conjunction þe (see the). The þy was dropped and the remaining two words contracted into early Middle English leste.