Английский Этимологический словарь - quarantine

- 1523, "period of 40 days in which a widow has the right to remain in her dead husband's house." Earlier (15c.), "desert in which Christ fasted for 40 days, from L. quadraginta "forty," related to quattuor "four" (see quart). Sense of "period a ship suspected of carrying disease is kept in isolation" is 1663, from It. quarantina, lit. "space of forty days," from quaranta "forty," from L. quadraginta. So called from the Venetian custom of keeping ships from plague-stricken countries waiting off its port for 40 days. The extended sense of "any period of isolation" is from 1680.