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 ... countries waiting off its port for 40 days (first enforced 1377) to assure that no
... [DOC] [TLE] Unquarantined | Define Unquarantined at Dictionary.com [PAR] Unquarantined definition, a strict isolation imposed to prevent the spread of 
disease.