Definition of lest in English:

lest

conjunction

formal
  • 1With the intention of preventing (something undesirable); to avoid the risk of.

    ‘he spent whole days in his room, headphones on lest he disturb anyone’
    • ‘Those in attendance are asked not to look directly at the panelists, lest you disturb the mood.’
    • ‘They are sent to sleep on the streets at night, lest they disturb the mother's business.’
    • ‘They usually prefer to keep their goals as vague as possible lest it count against them at the next election.’
    • ‘As to this it is necessary to avoid misapprehension lest the protection be too limited.’
    1. 1.1(after a clause indicating fear) because of the possibility of something undesirable happening; in case.
      ‘she sat up late worrying lest he be held up on the way home’
      • ‘When she was a couple of feet still from the bed she stopped, afraid to go on lest her fears came true.’
      • ‘Portia immediately fell in love with him and feared lest he should choose the wrong box.’
      • ‘The economy suffers, as my sisters and brothers fear going to work lest they find a bullet in their mailbox.’
      • ‘I'm a bit worried about posting this, lest it is not taken as it is meant.’
      • ‘If I had been less cautious I might have been more wise, but I was half crazy with fear lest you should learn the truth.’

Usage

There are very few contexts in English where the subjunctive mood is, strictly speaking, required: lest remains one of them. Thus the standard use is she was worrying lest he be attacked (not lest he was attacked), or she is using headphones lest she disturb anyone (not lest she disturbs anyone). See also subjunctive

Origin

Old English thȳ lǣs the whereby less that later the læste.

Pronunciation:

lest

/lest/