Originally, it was a 40-day-long period of imposed isolation, so it's from the Italian for "forty"
... a period, originally 40 days, of detention or isolation imposed upon 
ships, persons, ... Upper Italian (Venetian): period of forty days, group of forty, 
derivative of ... plague-stricken countries waiting off its port for 40 days (first 
enforced 1377) to... [DOC] [TLE] Last Words on Climate - Crosswords For Congress [PAR] Dec 16, 2015 ... "If I ____ so myself ..." 72.