Quarantine is a word that arose during the time of the Black Death in Europe and describes a period of 40 (quaranta in Italian) days in which ships from plague-affected areas were held at anchor before being allowed to come ashore.
In the 14th century, the Black Death (bubonic plague) was ravaging Europe. Cities were looking for ways to protect their citizens.
Officials in the Venetian-controlled port city of Ragusa (now Dubrovnik, Croatia) passed a law establishing trentino, or a 30-day period of isolation for ships arriving from plague-affected areas.
This practice as adopted by other coastal cities, and within a century the period of isolation was extended to 40 days. The word trentino was then replaced with the word quarantino to refer to this period of forced isolation. The Italian (Venetian) word trenta, meaning 30, formed the basis of trentino and quarantino was based on quaranta, Italian for 40.
Further clarification comes from the CDC and elsewhere that indicate that the word “was derived from the Italian words quaranta giorni which mean 40 days.”
Strangely Merriam Webster offers an alternate origin, though the meaning remains as described above.
Initially, the French word quarantaine (“about forty”) was borrowed in the late 1400s with the meaning “a period of forty days,”