Definition of "lest"

I thought to haue told thee of it, but I fear'd / Leaſt I might anger thee.

1610–1611 (date written), William Shakespeare, “The Tempest”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act IV, scene i], page 15, column 2

And then Robert Loo came out swiftly with the half-filled jar lest more be said.

1959, Anthony Burgess, Beds in the East (The Malayan Trilogy), published 1972, page 565

Stay free from petty jealousies / Live by no man's code / And hold your judgment for yourself / Lest you wind up on this road

1967, “I Am a Lonesome Hobo”, in Bob Dylan (music), John Wesley Harding‎

Lest any astrologer reading this result get cocky, Dr Cajochen does not believe that what he has found is directly influenced by the Moon through, say, some tidal effect. What he thinks he has discovered is an additional hand on the body’s clock-face.

2013 July 27, “Lunacy?”, in The Economist‎, volume 408, number 8846

That you and I should be in the same house together and not able to speak to each other is in itself a misery, but this is terribly enhanced by the dread lest this state of things should be made to continue.

1869 May, Anthony Trollope, “Lady Milborough as Ambassador”, in He Knew He Was Right, volume I, London: Strahan and Company, […], page 81

[M]y ward, or rather my adopted son Leo Vincey and myself have recently passed through a real African adventure, of a nature so much more marvellous than the one which you describe, that to tell the truth I am almost ashamed to submit it to you lest you should disbelieve my tale.

1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887

Mr. Cooke at once began a tirade against the residents of Asquith for permitting a sandy and generally disgraceful condition of the roads. So roundly did he vituperate the inn management in particular, and with such a loud flow of words, that I trembled lest he should be heard on the veranda.

1897 December (indicated as 1898), Winston Churchill, chapter IV, in The Celebrity: An Episode, New York, N.Y.: The Macmillan Company; London: Macmillan & Co., Ltd.