The story
From patients claiming, genuinely or not, to be temporarily discharged from the Abraham ward at Bethlem Royal Hospital (also known as Bedlam), a psychiatric hospital in London, for the purpose of begging. Possibly an allusion to a story in Luke 16, in which the beggar Lazarus ends up in Abraham's bosom. First use appears c. 1561 in The Fraternity of Vagabonds (1561) by John Awdely.
Etymology from Wiktionary, CC BY-SA 4.0.