The Collaborative International Dictionary of English v.0.48:
Quarantine \Quar"an*tine\, n. [F. quarantaine, OF. quaranteine,
fr. F. quarante forty, L. quadraginta, akin to quattuor four,
and E. four: cf. It. quarantina, quarentine. See Four, and
1. A space of forty days; -- used of Lent.
2. Specifically, the term, originally of forty days, during
which a ship arriving in port, and suspected of being
infected a malignant contagious disease, is obliged to
forbear all intercourse with the shore; hence, such
restraint or inhibition of intercourse; also, the place
where infected or prohibited vessels are stationed.
Note: Quarantine is now applied also to any forced stoppage
of travel or communication on account of malignant
contagious disease, on land as well as by sea.
3. (Eng. Law) The period of forty days during which the widow
had the privilege of remaining in the mansion house of
which her husband died seized.
Quarantine flag, a yellow flag hoisted at the fore of a
vessel or hung from a building, to give warning of an
infectious disease; -- called also the yellow jack, and