1. For fear that, in order to prevent
   "she tiptoed lest her mother should hear her" 
2. In case
   "he worried lest he should be late"

Encyclopedia: Lest

• one-handed
• one-hitter
• one-horse
• one-horse town
• one-humped
• one-hundred-millionth
• one-hundred-thousandth
• one-hundredth
• one-liner
• one-man
• one-man rule
• one-member
• one-millionth
• one-night stand
• one-ninth
• one-off
• one-on-one
• one-party
• one-person
• one-piece
• one-quadrillionth
• one-quarter
• one-quintillionth
• one-seed
• one-seeded