beggars can't be choosers

The substitution of can't for must not is a recent development. Cf. mid 15th-cent. Fr. qui empruncte ne peult choisir, he who borrows cannot choose.

Folke say alwaie, beggers shulde be no choosers.
[1546 J. Heywood Dialogue of Proverbs i. x. D1]

My Lords, says I, Beggars must not be Chusers; but some Place about a thousand a Year‥might do pretty weel.
[1728 Vanbrugh Journey to London iii. i.]

Crawl out o' that bed! I'spose you do feel a little bad, but ‘beggars can't be choosers!’
[1888 N. J. Clodfelter Snatched from Poor House iv.]

‘I suppose‥you would marry any man with a good character and a fine estate.’‥‘Beggars can't be choosers, you mean!’
[1939 J. Shearing Blanche Fury 72]

It would ruin the dress, no doubt about that. But beggars couldn't be choosers. She drew a breath and then jumped, tucking and rolling as she came out of the train.
[2000 J. Altman Gathering of Spies ix. 150]

Related to: necessity; poverty

Bibliography of major proverb collections and works cited from modern editions is available here.

Those in dire need must be content with what they get. For example, The cheapest model will have to do--beggars can't be choosers. This expression was familiar enough to be included in John Heywood's 1546 collection of proverbs.