lest

The story

1. Old English · inherited fromþȳ lǣs þe“whereby less that”
2. Middle English · inherited fromles te“less that”
3. English todaylest

c. 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.