Lest/lest/ Definition 1 of 2Pro Tip 1/3Cautionary Use Use 'lest' when advising caution to avoid negative outcomes. Speak softly lest you wake the baby. Pro Tip 2/3Triggering Action 'Lest' often precedes actions taken to prevent an unwanted scenario. Save your work regularly, lest the computer crashes. Pro Tip 3/3Emotional Intensity Using 'lest' can add a sense of urgency or importance to the caution. Stay silent, lest we are discovered.