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, ... 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 
assure... [DOC] [TLE] Quarantined | Define Quarantined at Dictionary.com [PAR] Quarantined definition, a strict isolation imposed to prevent the spread of disease
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