Certain monks and nuns, who use sandals instead of shoes. The Jews and Romans used to put off their shoes in mourning and public calamities, by way of humiliation. The practice is defended by the command of our Lord to His disciples: “Carry neither purse, nor scrip, nor shoes” (Luke x. 4).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894