quarantine

From Italian quaranta giorni, “forty days” — the wait Venice imposed on ships arriving from plague-affected ports in the 14th century before anyone could come ashore.

The word stayed tied to that exact span for centuries: forty days, no more, no less. Only in modern use did it loosen from the number itself, coming to mean any isolation period imposed to stop disease from spreading — regardless of how long it actually lasts.

Quarantine's root, quaranta, traces back to Latin quadraginta (forty), which makes it a distant cousin of quarter and quart — all ultimately from quattuor, four.