quarantine: [17] Quarantine denotes etymologically a period of ‘forty’ days. It goes back ultimately to Latin quadrāgintā ‘forty’, whose Italian descendant quaranta formed the basis of the noun quarantina ‘period of forty days’. English used it originally for a ‘period of forty days’ isolation’, but gradually the stipulation of the number of days faded out. => quarter[quarantine etymology, quarantine origin, 英语词源]