The word quarantine comes from the Italian for forty days.
During the plague epidemics of the 14th century, Venice made ships wait 40 days, a quaranta giorni, before anyone could come ashore. The practice was first introduced in the port of Ragusa, now Dubrovnik, as a 30-day trentino.
Why is this true?
Forty days was long enough for plague to show itself among the crew, and it echoed biblical periods of 40 days. The word has kept its meaning for six centuries even as the number of days changed.
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Language facts show how much of everyday speech is historical accident. Words that once meant the opposite, letters that vanished from the alphabet, punctuation with names almost nobody uses. Each entry explains the change that produced the oddity — a sound shift, a printing convention, a borrowing from another language — so it works as a genuine explanation and not just as a piece of trivia.